I've done this way back in High School. I don't think it's just people who stalk you. I think it's more of your interactions, people you talk to, people with a lot of posts you 'like' and comment, people you stalk, people who stalk you etc.
So it could just be you excessively stalking the person, that's why they're there.
That's what I think LOL I used to always check this back then