This might be one of Twitter's automatic security features I'd heard about a while back (prior to the Musk takeover). Supposedly, Twitter stops accounts from getting a large amount of followers in a short amount of time to try to limit botting. Sometimes, rarely, it can trigger when a large amount of real people follow a real account.

I think I heard about it when I ran into the issue myself, but I don't remember what account it was that I was trying to follow.