Well, one obvious explanation for people thinking themselves trans is that maybe one of the secondary sexual characteristics flipped or maybe even something not related to sex at all. As a result they don't quite fit in, then they here about the transsexuality movement and convince themselves that they've found the problem.

Note that this is consistent with the twin studies. All the brain scan stuff strikes me as the "we picked it up on a brain scan therefore it must be biologically caused" fallacy.