Maybe it's because I'm an autist but the second I turned 10, 11 or so it felt like pretty much all my interests were suddenly called babyish or "inappropriate for a girl your age". I'm talking stuff like dinosaurs, anime, pokemon, etc, stuff you wouldn't think is terribly weird for a kid that age to be into and people seemed to tolerate me liking before. It just got worse as I got older, and I've heard about other women, especially autistic women, talking about experiencing similar things. That's my personal hunch on why autistic girls in particular are so prone to trooning out, we tend to get particularly strong pressure to conform to gender stereotypes especially as we age from parents, teachers, and other non-autistic "autism specialists".
It's not something that happens to everyone, but it's something that seems to happen to a lot of people and I don't doubt is part of why so many girls are chopping their tits off and calling themselves he/they before they're even old enough to vote.