It's been around a long time.
Its massive use in adolescent girls has come out of nowhere. There is no proper research saying that it has X benefits for them.
If there were lots of women in the 25-70 age bracket all saying, 'Huh, I always thought I was really man, gimme some of that', I'd be thinking differently. I can remember being gay becoming more acceptable and all sorts of people gradually coming out of the closet, so you'd expect to see the same here if being trans is something that endures. But you're not.
Which is why I think it's a social contagion.