That is really odd. I'm really sorry this happened to you, but I must say that I am as perplexed as you are on legit possible reasons.
My suggestion is to talk to him and ask him what's up. If he refuses to tell you, then it's probably best just to drop it and break it off with him for good. If you both can't be open in a relationship, then it's not a good idea or healthy to still be in that relationship.
I personally doubt that it had anything to do with you. I think that it's just him who has the problem. But if he does say that it's because of you, then don't feel too bad about it. It's his fault that he didn't talk to you about it. You're not a mind reader. How were you supposed to know if it was "your fault" or not? I'm only saying this because I want you to be aware of this possibility, not because I think it was.
Good luck.