Sounds like he may have reached around with his left hand (if he carries on his right side)
to hold the holster in order to 'horse' the gun out, then shot his hand.
`Never let your muzzle cross anything you don't intend to shoot.' Jeff Cooper
If one places his finger on the trigger WHILE DRAWING, the weapon will fire. (Don't try it, but you can see why.) This should be a slam dunk for the defense.
The article is short and doesn't indicate the EXACT circumstances of the negligent discharge. Was he drawing or reholstering? Some use the weak hand (LEFT if you are a RIGHTY) to hold open the holster while reholstering. This unfortunately results in SOME WHO HAVE BAD/UNHEALTHY TECHNIQUE TO COVER THEIR LEFT HAND WITH THE MUZZLE.
Now, these failings in isolation would not be critical. One must violate NUMEROUS safe-handling laws to SHOOT THEMSELVES.
Let's assume he DID NOT have his finger on the trigger. I have seen ND due to a SHIRT TAIL in the holster while reholstering. But in this case the weak hand will NOT be covered. The body part covered is usually the buttocks.