Maybe he was in a fight.
Check out this beauty I found on a hike last fall, right along a game trail into a meadow I was scouting for later that season.
The left antler was broken but so was the base. You can see how it healed at an angle. My guess is it was a bad head-butt and a heck of a headache! There's significant bone growth around the base of the antler where it broke - it was the skull that broke, not just the tines on the antler itself.
As you can see the skull is small and the right antler has just the two tines you see. I made him a 1-1/2yr old buck. Since the left antler is clearly broken, my belief is the injury was caused the same year I found the skull. But I found the skull in mid October and there was significant bone growth and healing around the wound, so I don't believe it was a rut fight, I think it happened earlier in the summer. I'm no biologist but that amount of healing would have to have taken at least a few months IMO.
We know bucks fight most in the rut but they can fight any time. Maybe yours was a pre-drop battle. Or maybe he got his antler stuck in something and ripped it off. I know people sometimes make "antler traps" that get bucks caught and force them to drop which IMO is cruel and I'd never do that... but what do I know. But what if yours had a fight or just got stuck in a tree?