I've had this happen before myself. Manga that I'm following will randomly unfollow, or have all the chapters that were read appear unread. It happened shortly after I first signed up to the site, then it appeared to go away for some time, and now just the other day it's started back up again. Just found another series unfollowed a couple of minutes ago, and another series unread before that. Yesterday I think it was 3 series I found unfollowed, and 2 completely unread.