Years ago, when Glocks were first coming onto the scene, I was working the ER. We had a Sheriff's Deputy come in with a .45 Black Talon round on the lateral condile of his right knee from his issue Glock 20. He claimed that the "Gun without a safely just went off". After some Morphine for the pain, and the additional fact that the guys working on him were more familiar with the Glock than he was, we got to the root of the problem. IA just happened by about the time that the Morphine was really kicking in as well.
The holster he was using had a retention safety strap designed to go around the rear of the slide when the gun was securely holstered. The Deputy was re-holstering his weapon, in an office mind you, when upon meeting resistance, he pushed down harder. The retention strap had entered the trigger guard and engaged the trigger. Add the extra downward force and BOOM! Hello knee surgery. He managed to engage the bang switch without even using his booger hook.
There are ways where the "holstered" M&P.40 in question could have gone off but they are extreme, as in the case above and, it points out that it doesn't happen without somebody making it happen.
As to how the round traveled UP from a waistband holster is another mystery all together. With Detroit style law enforcement in action, we should promptly have all the facts in about 120 years.
Doc