It’s just bots or spammers generating accounts on various instances (<- edited for clarity and correction) then using them to spam popular communities. Happens pretty regularly, but unfortunately if the home instance doesn’t ban the user then each instance has to remove the spam themselves.

Also, there’s no way to report a user to their home instance so long as they don’t post anything in a community on their home instance. So the only people who can ban them in a way that federates to other servers also doesn’t know unless someone sends them a personal message. And they are using a bunch of different instances to make it hard to do that.