They were likely a trock: a “sock puppet” account created by a previously-banned poster, in order to return and troll the board. Creating a second, “sock puppet,” account, specifically for the purpose of making posts without letting on that you have (or previously had) a different account here, is against board rules, and is a bannable offense.
We regularly have banned posters try to come back, under different account names, nearly always just to stir up shit.
My educated guess is that one or more people flagged that now-banned account’s posts to the moderators; the moderators then were able to determine that it was a sock/trock, and banned it.
When the mods discover that a poster is likely to be a trock, they ban the new account, and move the posts made under that new account to a thread that’s not visible to non-moderators. In essence, any posts made by the trock vanish from view.