Not the easiest weapon to have an ND with and usually involves pulling the trigger in some way, not always with the finger. Just one example - officer while wearing his over jacket, holsters his Glock trapping the plastic toggle in the trigger guard within the holster, turns & feels his jacket caught on something, gives it a tug & bang 9mm round into leg. However in this case posted above I have no knowledge of the circumstances and would not speculate as to the cause.
Wingy