I don't think anyone has mentioned it, so I will. We keep talking about holster or not here (which i'm a fan of), but there's one more thing worth repeating. While it's not 100% clear that this happened, there's enough evidence to think it MIGHT have been the case that he caught the gun while it was falling.
As he reached for the weapon, it went off, blowing a hole through his pants.
Most of us already know it, but don't catch that falling gun. It's infinitely more likely to go off if you catch it than if you just let it hit the floor.
If that's not quite what happened then so be it, but I'll throw that out there anyway.