In the case reported in this thread, you would think that the gun would be more likely to accidentally discharge upon reholstering rather than while actually being worn normally
The problem with this type holster is, youre constantly reholstering/reseating the gun at some level with them, just wearing them. I spent some time with a similar holster early on, and I swear, I had more trouble with it than I ever did, just carrying the gun Mexican style. Youre constantly reseating the gun in the holster, and many times, catching it as it comes completely out.
From looking at the pics, Id be willing to bet, he sat down, the gun got pushed up, he reached back, shifting a little and pushed it back down, slightly off from the way it came out, and possibly even catching the edge with the muzzle, causing it to cave inwards.
A proper, fitted holster, with no straps, and a solid, fixed opening, that covers the whole gun below the grip, is your best bet safety wise.
Another thing about reholstering an automatic pistol is the tendency for the slide to push back if it is a tight fit and the pistol does not have a locking safety like a Colt Government Model.
That would have actually been a plus in this case, as the gun would not have fired if the slide was out of battery, and even if it was physically pushed forward after the fact, the trigger would have to be reset and pulled to fire the gun.