I think what happened was they merged your Youtube Subscriptions with your Google+ Subscriptions. I had a bunch of people who I unsubbed a long time ago resub because of this. I'd imagine this is what happened to you. Once agan Google + Fuckes everything lol,
Hmmm. Idk. Like I said, everyone that showed up in my sub box i had been subbed to before & unsubbed at some point. Because I never touch google+ it makes sense. But if that's not it then there must be something else going on too.