Guy ended things without letting me explain — need advice on what this means

I was seeing a guy for a few months. It started casual and we were physically involved, but over time we both caught feelings. Even though he was hot and cold because of his work schedule (he’s away for 2–4 weeks sometimes), he would still say he missed me or loved me. Recently, something from my past came up. It didn’t involve him and didn’t happen while we were seeing each other. But when he found out, he ended things immediately, didn’t let me explain, and blocked me. I’m confused because I didn’t betray him or lie to him. His reaction felt sudden and extreme. My questions: What makes someone end things without even hearing your side? Is this a red flag or just insecurity? How do I move on when it feels unfair?