The cop either:
A. Holstered his weapon with the coat caught in between the holster and weapon, thus his error, or:
B. Holstered his weapon with his finger on the trigger, again, his error.
No jacket zipper or "toggle" can work it's way into a quality holster, or for that matter, a cheap holster that covers the trigger.
And before Glocks were king of the hill for police weapons, the leader of the bunch for ND's was the 1911. I've seen several holes in a gunshop from cops unholstering their 1911's. Nothing new there.
I state again, if you don't pay attention, you will get bit.
That's the way ADs during holstering always happen, isn't it?