Involuntary Group Split

The characters are in a cave, or occasionally a building, and after a cataclysmic event, the falling pieces of the structure separate them into smaller groups. They must either find their way out alone, or if some of them have no way to escape, they must be rescued, often with a race against time.

Very common in video games, as a plot device for separating the player character from its party, because in single-player games, too much help from friendly NPCs would take away the meat of the game. Briefer examples are simply an Inescapable Ambush.

Examples of Involuntary Group Split include:

Anime and Manga

This happens to the group in Mahou Sensei Negima both figuratively and literally; the Big Bad used a falling stone spell to smash a teleporter, the chaotic magic from this scattering them all across the magical world.
- This also happens later on during the attack by Cosmo Entelecheia during a formal party held by Godel, as a collapsing structure causes Nodoka and Asakura to be separated from Negi, Chisame, and Ku Fei just as Nodoka is about to tell Negi the final secret to the Magic World.
It happens in Pokémon every once in a while. Though it's more of a party mismatch, since generally, Rockets, trainers, and even Pokémon can form their own groups after the rocks fall.
Repeatedly used in Digimon Adventure. First Devimon splits File Island, then everyone has drifted apart by the time Taichi finds his way back to the Digital World after Etemon, then they choose to separate during the Dark Masters arc (during which one of the split groups was briefly split even further when Mugendramon blew up a skyscraper on them, casting them into different parts of the city's sewers) and don't completely regroup until the absolute last minute.
- The Digital World arc of Digimon Tamers had this as something of an environmental mechanic: the data streams of the lowest plane of the Digital World would drag those caught to a completely different plane of the World; on the other hand, they eventually used a data stream to regroup.
- Digimon Frontier literally has the party split because rocks fell. When part of the mountain they're on is eaten by Gigasmon to increase his power, the team falls...and winds up in different parts of the digital world instead of splattered.

Comic Books

Mouse Guard: Winter 1152 has Kenzie, Saxon and Sadie fall into an underground cavern, forcing Liam and Celanawe to continue without them.

Fan Works

A variant in With Strings Attached. When the four are in New Zork, they walk along unconcerned that they might be recognized, since it's 1954 and in Real Life they didn't show up in America until 1964. Unfortunately, this Earth is hardly an exact parallel with theirs. They round a corner and come upon a line of kids waiting to enter a contest to win “Beagles” tickets. “The mob broke the sound barrier as they thundered into the road, and the four said 'Christ!' and fled in four different directions.” The remainder of the chapter consists of their attempts to get back together.

Film

In Conan the Destroyer, Bombaata causes one of these to get rid of Conan temporarily. Big Mistake.
Happens in the Hellboy film with walls that come out of the ground.
A variant occurs in the Rollercoaster Mine in the 2008 version of Journey to the Center of the Earth. The handcart and the two mine cars break off at a track intersection, sending Trevor down one track (nearly crashing him into a wall), Hannah down another (that abruptly ends and almost plunges her into a ravine), and Sean down yet another (he comes out just fine).

Literature

Harry Potter and the Chamber of Secrets : When the tunnel leading to the Chamber of Secrets collapses, Ron Weasley and Gilderoy Lockhart are trapped. Harry must now face Voldemort alone.
In the Isaac Asimov short story "Catch That Rabbit", the heroes decide to create a fake rockslide, to test whether a robot is malfunctioning or not, but then get caught in it and have to get the robot to dig them out.

Live Action TV

Star Trek: The Original Series episode "Devil in the Dark": While Kirk and Spock are hunting the Horta deep underground, the creature causes a collapse that splits them up. After the Horta appears to Kirk, Spock eventually reaches him.
Happened in Lost after the characters discovered the cave.
Doctor Who episode "The Doctor's Daughter". Martha Jones is separated from the Doctor by a tunnel collapse near the beginning of the episode and meets up with him again near the end.
Happens to an extreme in season 1 of Stargate Universe. A small group are split off from the main cast and stranded on an alien planet when a cave-in traps them in an underground complex. Then, next episode, another cave-in splits them up, leaving one stranded unconscious whilst the others try to use the stargate network to make their way back to the Destiny. Possibly the first time a cave-in has left the party split up across the breadth of a galaxy.

Tabletop RPG

Star Frontiers adventure SF0 Crash on Volturnus. While the PCs are in the Forbidden Caverns with the Ul-Mor, a cave-in will separate the two groups and leave the PCs on their own.

Video Games

Warcraft: The Frozen Throne's undead campaign has a level where Arthas is separated from Anub-Arak by a convenient earthquake and must navigate the maze alone. At level 2. While the clock ticks down. Fortunately, Anub-Arak digs his way back to Arthas in time.
- In the Night Elf campaign where, causes a cave-in which effectively separates the Warden from the rest of her party because only she has the teleportation ability required to escape otherwise certain death.
In Half-Life 2, Gordon is separated from Alyx this way, when Black Mesa East is attacked
In Destroy All Humans! 2, Natalya and Crypto are split up by falling rocks in a secret volcano base in a cheap Expy of Japan.
Happens all the time in the Kingdom Hearts games, usually to force you to fight bosses alone. (Usually without explanation too - a forcefield just zips up when the boss appears. Though sometimes there are better reasons.)
Happens in Eternal Sonata when a boss villain destroys the bridge on which you're standing. Half of the party is rescued by a passing ship and the other half washes ashore in a swamp. Conveniently, the villain doesn't end up with either group and is never heard from again.
The Guild Wars Factions mission Arborstone has a cathedral begin collapsing once the Urn of Saint Viktor is removed. Mhenlo gets out before the front door is blocked off, but all the others have to work their way to the back exit.
Resident Evil 5 has a thankfully brief case of this in chapter 4-1.
In Resistance, the ceiling collapses into a pile of rubble, separating the British soldiers from Nathan Hale.
After a Hopeless Boss Fight in Star Ocean: The Second Story, the heroes are kicked overboard. After washing up on the beach, Rena discovers Claude and Leon are missing, along with all of the NPCs that were on the ship, and struggles to keep calm. Meanwhile, Claude wakes up on a different part of the beach and has to deal with Leon crossing the Despair Event Horizon.
Leon and Claire are split up by an exploding tanker truck in the beginning of Resident Evil 2 and each is forced to make their way to the Raccoon City police station alone.
- Also happens a few times in Resident Evil Zero. Since partnership is important in that game, it's all the more troubling when it occurs.
In Super Paper Mario, Mario and company get separated after Dimentio blasts them all to the Underwhere.
In Fire Emblem, the Morph Kisuna ensnares the player's party and uses his magic to divide them into two groups.
In Silent Hill Homecoming, this happens in the prison level almost immediately after meeting your companion.
In Head Over Heels, after spending some time getting the two characters together, the game continues to come up with inventive ways of splitting them up again.
In Exile/Avernum 3, this happens a few times, most notably when defeating the Slime and Cockroach plagues.
Subverted in Final Fantasy V. The party falls down a hole, with three of them waking up in a large room and the fourth in a small closed-off chamber nearby. After some brief discussion the majority decide to carry on by themselves and assume their missing party member will catch up with them later on. Cue the man himself popping out of an underground passage to berate the party for planning to leave without him.
The first two Mario & Luigi games.
In Black Sigil, not long after entering the House of Black Stone, your party of three will be broken down, with Kairu alone and Asi with Aurora. It only takes a few minutes before you're together again, though.
Happens briefly in Xenoblade Chronicles, after the destruction of Galahad Fortress causes the party to fall into the ocean.
Mega Man X Command Mission has this happen at the very beginning. An explosion separates X from Zero and Shadow, but they can still communicate with each other.

Western Animation

Avatar: The Last Airbender has the Season 2 episode “The Cave of Two Lovers,” in which a cave-in causes Aang and Katara to be separated from Sokka, leaving Sokka with the hippie musicians.
My Little Pony Friendship Is Magic does this in the first episode of Season 2, with the mane six getting separated by one of Discord's pranks.