I'd say the sunspots
As while there might be an increase in that more people 'seem' to be TG than before that could also be because society is more accepting than before.
There have been TG people in the past, in ancient history. Some were quite well regarded in their societies becoming shamen or priests/priestesses (or divinely inspired warriors like the burnt-for-wearing-pants-and-short-hair Joan of Arc) so whatever the cause it can't be wholely something recent.