The Wikipedia article on it is informative. https://en.wikipedia.org/wiki/Rapid-onset_gender_dysphoria_c.... In short, it's a hypothesis that gender transition is a social contagion that has been rejected by basicly every organization that knows anything about trans people and is mainly a way to dismiss the agency of transgender youth.

The evidence that's supposed to support it can be better explained by the alternative hypothesis I've seen called "ROPKAI", or "Rapid Onset Parents Knowing About It".

It's interesting how often green accounts appear in the comments of these articles to say stuff like what's in the GP.