I’ve had the same thing happen to me. Most of the new follows are low-rep, and avatar-less accounts. It appears to be a somewhat organized effort, to follow a lot of people, hope some follow back, and then hope they upvote the content they post.

Almost all of their posts consist entirely of links to YouTube videos, which Steemit displays as a video. I’m pretty sure they don’t actually make the videos they post about. However, this isn’t immediately obvious to people who blindly upvote everything, and follow back everyone.