Am I right to want an explanation?

I used to hook up with this guy, we developed feelings for each other, and we dated for a few months. The relationship ended because of distance, but we remained good friends and talked often. Recently, we were texting each other, just talking about life. I mentioned that I had hooked up with someone, and that I liked someone else. The conversation continued for a while, until he suddenly texted me saying he wanted us to stop talking, and basically, "goodbye." He blocked me on all social media and my number, but I can email him.

He didn't give any reason for cutting off contact, but my gut feeling is that he was upset about the fact that I had moved on. I really need to hear from him why he decided to cut things off, but I don't know if he would respond to an email I send him.

What should I say in an email to make it likely that he responds? I think I have a right to know why he decided to break off our friendship, so I am set on emailing him.
Am I right to want an explanation?
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