Predator/WMG

Predators don't come to Earth to hunt us for sport, but as a training exercise.
To train their soldiers, predators drop them on primitive planets filled with dangerous native creatures (with a limited set of equipment/supplies), then return after a set amount of time (a few days/weeks/months) to pick them up and see if they've survived. They actually do this all the time, but we don't notice because of their cloaking technology, and because most preds don't actually go after humans. The few that do (like the ones in Predator and Predator 2) are in the minority, and are considered insane by most of the others. Oh, and the ship full of preds at the end of Predator 2? His training exercise was just about to end, and they had come to pick him up.

The Predator homeworld is Pandora.
Predators breath a different atmosphere than that found on Earth - Pandora's atmosphere is very different. Many creatures on Pandora display bio-luminescence - as does Predator blood and the healing material used in Predator 2 (herbs from home perhaps?). Predator society seems to revolve around who can kill the biggest toughest thing - which could actually be a survival trait on a planet full of absurdly deadly wildlife. Predators are excellent climbers, utterly at home in the treetops - much like the Na'vi. Predators clearly have anti-gravity technology worked out - which you would expect when they're surrounded by unobtanium.
 * Definitely plausible. The helmets that they wear look as though they're made of unobtanium.
 * Earth's atmosphere is similar to Pandora's which would explain why the predators can go without their masks for some time.
 * Their weaponry is certainly overkill for 6' bipeds.
 * Their tendrils could be hiding one of those Pandora 'mind meld' tentacles beneath them, who knows?

Predators is an adaptation of Team Fortress 2.
Each film has nine central characters, most of which correspond to and share multiple characteristics with each other.
 * Isabella - Sniper
 * Nikolai - Heavy
 * Stans - Scout
 * Cuchillo - Pyro
 * Mombasa - Demoman
 * Noland - Soldier
 * Royce - Engineer
 * Edwin - Medic
 * Hanzo - Spy

Predators are so driven to keep their tech out of alien hands...
Because they fear that if a race like humans got ahold of their technology, that race would be able to reverse engineer it, or at least to create viable defense and counter strategies, robbing the Predators of their biggest advantage. The reasons that humans are almost as favored as prey as Xenomorphs are (according to expanded universe materials, the Predator term for "human" is "Pyode Amedah" - "Soft Meat", in contrast to the "Hard Meat", or "Kainde Amedah" of the Xenomorphs) is because of their intelligence and their ability to think, strategize, and, from an intellectual standpoint, meet the Predators on equal terms. Give humans access to Predator cloaking devices and energy weapons (or, perhaps worse, detectors that can penetrate cloaks and armor that can resist energy weapons), and that puts the two races on terms much too equal for Predator liking.

Predators realize other races could steal their tech and overcome them because they did the same thing.
Predator technology wasn't originally developed by them, but was taken from another race (possibly the Space Jockeys/Pilots) and reverse engineered, catapulting them far ahead technologically while they were still in a rather primitive stage socially -- roughly comparable to the Krogan from the Mass Effect universe, who are at one point described as wha would happen if one gave nuclear weapons to cavemen.

The regular predators and "berserker" predators are the male and female of the same species
I am not sure, however, which one is which. Some predator expanded universe has portreyed females as much larger and tougher, but those aren't really canon, and the flamboyant features on the Berserker predator could be seen as a masculine symbol in the same way as a Male Peacok's feathers, although that very speculative.
 * Would that mean the "Blood Feud" between the two is actually just a bizarre (and unusually violent) mating ritual?
 * That would put new context on the scene to the normal predator who was overpowered and tied to that pole.

Jerry Lambert, who dies in Predator 2, is a distant ancestor of Pvt Hudson.
They're both played by Bill Paxton, and the films are related.