Back in the good ol' days, we had at least one MP per month, Army-wide, have a negligent discharge with a .45. This usually occurred while clearing the weapon and went something like this:
1. Rack slide to eject round in chamber.
2. Eject magazine.
3. Pull trigger.
4. Oh, s..t!!
If luck was with them, the only injury was to the floor, wall, or ceiling. In far too many instances, luck was not with them and they either seriously wounded or killed themselves or a fellow MP.
If the weapon in this instance was a semi-automatic, I would suspect that the clearing sequence I posted above was followed.