Hmm, how many people remove a slide with the muzzle pointing at them ? I don't !!
A retired cop who carried a revolver his entire career and had recently been given a Glock?
Actually, I can picture how it might happen. When I remove the slide from my G19, I hold the gun in my right hand with my thumb on the backstrap and my fingers around the slide. That makes it easy to squeeze the slide back just enough so that I can work the release tabs with my left hand. From that position, I can picture him seeing the trigger in the charged position and swinging his thumb around into the trigger guard to pull it back while holding the slide in his left hand. (Or the other way around if he was a wronghander.) The barrel would go from pointing down and away to straight at his chest.
All it takes is a split second for a lapse in gun safety training to ruin your whole day.