I am not a big fan of the Glock-type "safe action" setup as being the only safety on an auto. That's why I think the grip safeties are a good idea on the XDs.
But anyway, I'm guessing the weapon slipped down his pants and when he tried to stop it from falling he snagged the trigger.
If the pistol had a manual safety that was engaged - probably it would have lessened the chance that it would have gone off, if it had a grip safety - probably it would have lessened the chance that it would have gone off, depending how the gun was grasped, but like I said, i'm guessing he just grabbed for it and his finger went into the trigger gaurd snagging the trigger.
As far as the legal technicalities, I guess lawyers and union reps can argue about that ad nauseum, but the fact is - he did discharge his weapon.