My agency had a guy shoot himself in the leg putting a gun back in a holster. He still doesn't know how it happened. Shouldn't be possible because our policy is to have our thumb covering the hammer as the weapons is put back in the holster. If you hit an obstruction with the trigger you would feel resistance from the thumb holding the hammer down.
Shooting yourself while drawing a gun probably means this deputy has had bad habits for a while and his firearms instructors haven't been watching when he is at the range.