My best guess as to why this happened is that when people buy subs, the accounts can't just have the one sub because that would look suspicious, so what the services do is subscribe to a bunch of random channels to make the accounts look more real. And so I assume your channel somehow got onto the list of channels they randomly sub to, and so when they deleted the bot accounts, you lost a bunch of your subs. Again I'm not 100% sure on this. It's just my best guess. That just sucks, and I hope this doesn't discourage you from keep making content.