Maybe someone followed you initially just so you would follow them, then unfollowed you. Or maybe somebody decided to go through and unfollow a lot of people just to weed out their list. I’ve read Medium posts from popular writers recommending this, but I never do it because I don’t want to weed out my list. Like you, my followers are important. And what if i accidently unfollowed someone who really liked my work? For whatever reason my numbers of followers go up and down some and always have.