Another possibility to be considered is whether any smaller caliber ammunition was at hand that might've been loaded into the magazine.
For example in this case was there any 9mm on the table, or might a 9mm round have been mixed in with the reloaded .40's? (A wrong caliber round can sometimes find its way into even a box of new factory ammo.)
I've seen occasional instances where someone mistakenly loaded a .40 in a .45 magazine, or a 9mm in a .40 magazine. Sometimes the extractor might catch and hold the undersize round, and the case gets ballooned and split when the firing pin hits the primer, or, I've seen someone try to chamber what turned out to be a .40 in a .45, and watched the whole unfired cartridge fall out the muzzle.
Now, imagine for a second what can happen if the .40 round gets pushed forward of the chamber and is just sitting inside the bore when a .45 round is chambered and fired. I've handled a plastic .45 that had suffered significant damage because a .40 was lodged in the bore when a live round was fired behind it.
I've also heard that it's not exactly a rare thing for a gun company to learn of this happening with one of their products.
Sometimes it's difficult (or impossible) to determine if the wrong ammunition was being used, and sometimes it's possible when the evidence remains inside the bore.
Not intimating this happened in this case, but it's happened before, and it'll probably happen again. Something to consider, and a good reason to carefully inspect each round being loaded in a magazine, regardless of what it might say on the box or bag label.
The important thing is that the person using the gun, and anyone around him, weren't seriously injured. Guns can be replaced.
FWIW, I remember when we first transitioned to the early 5903's/6906's. Budget constraints forced us to use commercially produced remanufactured LRN 9mm ammo. I was on the range one day when one of our guys shooting his new 5903 had a problem with a round that blew out a case. It stung his hand a bit, and I think it may have caused the magazine to drop. He was pretty startled (and not a little agitated), but fortunately he was uninjured and the gun was undamaged.
It was immediately used as an example of why we needed to spend the little extra money it cost to buy new factory ball for training & quals. We got the money.