The reason the threads discussing trans issues/self-ID are spreading across Mumsnet, and other sites, is because people are finally recognising that there is an issue that needs discussing.
In addition to this, the reason you see so much discussion here is because MN is one of the very few places where you're even allowed to discuss this without being deleted, banned, shut down, etc. Most places online will quickly censor you if you dare to disagree with any of this, no matter how politely and respectfully you disagree.
A poster on another certain popular UK based forum was banned and had her sensible posts about how it doesn't matter if male brains and female brains are different because 50% of the population will still be discriminated against and oppressed because they have a vagina. Meanwhile posters posting misogynistic and homophobic crap are allowed to stay.
Tbh the fact that people who disagree with this are being no platformed and silenced just speaks volumes and was actually one of the things that opened my eyes. Why aren't we allowed to discuss this? Can you think of any other proposed law changes that people weren't allowed to discuss and only one side of the argument was allowed to be presented? Because I can't.
If we look at when same sex marriage was being discussed and debated for instance, people who disagreed with it were still allowed to speak and get their views across. Obviously I completely disagree with these people but nobody told them they should shut up or tried to silence them. All views were taken into consideration.
Yet with this we aren't allowed to discuss it at all. It's actually very sinister.