If you go to his Twitter page, it shows he isn't following anyone. He even unfollowed his own mother and some of his close friends. He had to have been bored witless to embark on a mass unfollowing spree, his account was either hacked or this is some sort of Twitter glitch. It would not be the first time users have experienced Twitter unfollowing people for them.
I even encountered it myself when someone I would never unfollow asked me why I unfollowed them. It still doesn't explain the many fans that have complained about him unfollowing them on Instagram as well. If he really did unfollow all of his fans, he picked a really bad time to announce a tour and new album.