Why would a guy stop messaging me on and off?

My friend and I send each other facebook messages in spurts. He and I would write each other back and forth, usually taking under a day or two to get back to each other. But sometimes, I will send a message back to him, and he'll just stop replying. It's like walking away in the middle of a conversation. Then weeks later, he will start talking to me again. I really don't understand what this behaviour means. It's different if a conversation isn't going anywhere and they're's not much more to say, but we will be in the middle of asking and answering each others' questions and he'll stop just like that. We aren't talking about anything sensitive or serious. It's very weird and feels unnatural to cut off communication in the middle of things. We were in a previous relationship, and he started talking to me earlier this year after no communication for over six months. I do not know if he still has feelings for me. I've never had (or rarely heard of) communication issues similar to this. What do you think?