Transformation Causes

In any story in which a character changes form, regardless of whether they become a giant, an Incredible Shrinking Man, or even the opposite sex, this is usually explained using of the following reasons:


 * They Just Woke Up That Way.
 * A Mad Scientist
 * An Evilutionary Biologist experimented on them.
 * They were shot with a Transformation Ray or Shrink Ray.
 * They were Playing with Syringes.
 * They were injected with a Super Serum.
 * The Super Serum went wrong.
 * They were genetically modified.
 * Which gave them an Unstable Genetic Code.
 * They were involved in a nasty Teleporter Accident.
 * They were accidentally fused together with another creature.
 * They were accidentally exposed to a strange substance.
 * They ate the Mutagenic Food.
 * They ate the new and improved crunchtastic Yummypops.
 * They were exposed to strange radiation or nuclear waste.
 * They got too close to the Green Rocks.
 * They were in a strange place that caused transformations.
 * They took a dip in the Fountain of Youth.
 * They fell into the cursed swamp.
 * Their reality got warped or re-written.
 * They got hit by the World-Wrecking Wave.
 * They have always been destined to transform at a certain point in their lives.
 * They're a Half-Human Hybrid
 * It's a natural process of Metamorphosis for them.
 * They have a number of "natural" genetic mutations that don't manifest until puberty.
 * They inherited a family Curse.
 * They're "evolving" into a superior lifeform.
 * They were carrying certain genes that got "activated" somehow.
 * Transformation is contagious
 * It's a virus, and the character was infected (possibly through biting).
 * They were splashed with a monster's bodily fluids.
 * They became the host to an alien parasite.
 * They received a Face Full of Alien Wingwong.
 * They were by something that wanted them to transform.
 * They received the form from somebody else.
 * They were given the form as a gift.
 * They earned their new form by Working for a Body Upgrade.
 * They were given a Superhuman Transfusion.
 * A dying character gave them the power.
 * They were given a copy of the form from another character.
 * They gained the form by fighting someone with it (although the form may not last).
 * They assimilated or ate a foe who had the form.
 * They became subject to You Kill It, You Bought It.
 * They gained the form as a reaction to being attacked.
 * They were on the brink of death and it was the only way to save them.
 * They were "rebuilt" with cybernetic parts
 * They switched bodies with someone else
 * They triggered a Transformation Trinket.
 * They wore a garment that transformed them
 * A Clingy MacGuffin got stuck to them.
 * They made the wrong wish to a Literal Genie.
 * They pissed off a witch.
 * They pissed off the Gods.
 * They pissed off the author.
 * The author is into that sort of thing.
 * They were a complete jerk and they deserved it.
 * They were such a jerk that they started transforming all on their own.
 * It was a Bargain with Heaven.
 * Or a Deal with the Devil.
 * A Wizard Did It
 * They were struck by lightning.
 * They wanted to change.
 * They only think they transformed
 * A videogame hero has just slain their usual form.

It's also worth noting that a lot of these are also what might give a character Superpowers, however, gaining a superpower tends to be more of a plot focus, whereas in a transformation story, the focus is more on what happens during or after the change rather than what caused it.