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:

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.