Admins can see all votes of users on their instance, as well as all voter’s usernames on a post located in a community that’s on that instance. So if they get suspicious amount of votes from a user on another user’s post, they might consider that vote manipulation and ban either the voter, or poster, or both. (Although, banning a poster for receiving suspicious amount of votes can lead to wrongful ban, since someone could just make random alts to consistently upvote someone they hate in attempt to ban them.)

The admins probably saw suspicious votes and banned those users.