My understanding is that at least some of these instances are probably being attacked by bots that are auto-signing-up accounts on instances with open aign-up, because someone looking into it said that the instances are also banning a bunch of them. Probably not complicit (though that will presumably also be a long-term concern to deal with).
The instance admin at lemmy.dbzer0.com was apparently working on some kind of tool to monitor it. Had a start on GitHub, IIRC.