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 may have employed the Evolutionary Pressure Cooker to produce a single strong creature from many weaker ones.
- Or alternatively, a Science Hero experimented on their allies.
- They were shot with a Transformation Ray or Shrink Ray.
- They were Playing with Syringes.
- They were injected with a Super Serum.
- They were genetically modified.
- Which gave them an Unstable Genetic Code.
- An Evilutionary Biologist experimented on them.
- 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 ate the Mutagenic Food.
- 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.
- Certain conditions led to Diverging Evolutionary Phases.
- They're a Half-Human Hybrid.
- 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 died and Came Back Wrong, becoming a Monster from Beyond the Veil.
- They died and Came Back Strong, becoming an Empowered Badass Normal.
- They are afflicted with a condition or curse that makes them change into another form at night.
- They became the host to an alien parasite.
- They received a Face Full of Alien Wing-Wong.
- They were possessed 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.
- Another character didn't want to wait for their clone, so they chose another person.
- 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 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 (or a Jackass Genie).
- They pissed off a witch and got cursed.
- They pissed off the Gods and got cursed.
- 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.
- They earned the favour — or disfavour — of The Transmogrifier.
- 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 video game hero has just slain their usual form.
- They had sex or fell in love with a super.
- They get splashed with water.
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.
