When you've cared about someone, it's difficult to completely turn off and sometimes you wonder how they are, or you miss talking to them. Sometimes they just cross your mind. Other times, you might find yourself having feelings again, or realising you wish you hadn't broken up. It could be any of these things.
If you ask him in a friendly, open way (i.e. "Haha! What made you text me?") as opposed to a hostile way (i.e. "WTF are you talking to me for?"), chances are he'll tell you.