A lot of people, when they get a follower, will follow the other person back. However, the person initiating the following only wants them as a follower, they do not genuinely want to follow that person's content. So they wait a couple of days and unfollow the user, keeping them as a follower but not having to see any of their content. It's not really that beneficial since there is no connection of content with the new user, and the new user is therefore less likely to upvote content, but it's a shady tactic nonetheless.
RE: HERE ARE SOME PEOPLE STEALING FROM YOUR SUCCESS ON STEEMIT