This is... fairly normal; possibly people forget because we have been in a period of relative stability for social networks, but this is how it usually works. It's basically how web forums reproduced, when those were the primary form of social media; forum starts to get a bit shit, people get annoyed, communities within that forum start decamping en masse for elsewhere (often a completely new forum). It's a fairly natural human behaviour; if the people you interact with go to X, you probably go to X, too.
I mean, what do you expect to happen? 20% to Mastodon, 20% to Threads, 20% to T2, 20% to Bluesky, 20% stay, to avoid being perceived as a 'hivemind'?
This is actually one reason I think it's rather unlikely that there will be a single Twitter-killer, with everyone from Twitter moving to Threads or whatever and then continuing on as normal. Threads will probably be the biggest (particularly if it manages to launch in the EU; that's a big problem for it right now), but I'd expect a year from now that there'll be Threads, a bunch of Mastodon instances (maybe more loosely federated than today, probably more community-oriented), maybe Bluesky (I don't personally _get_ it, but some people seem to like it) and a rump Twitter.