Stop it tumblr!
This is a fact. I’ve noticed it, too. It goes both ways as well, with Tumblr having some of your followers “unfollow” you. I’ve been tracking it, even, and have pretty much come to the conclusion that my follower count has been fixed. If I get two new followers today, two others will unfollow me tomorrow. If ten people follow me, ten others will unfollow me. It’s pretty much like clockwork, keeping me right where I’m at. So, if I’ve unfollowed anyone, no, I didn’t. Periodically, I unfollow dormant blogs, but that’s it.