If it was the wrong powder, it would have shown up earlier, assuming this wasn't the first shot. And assuming he didn't load the shells with a dipper.
It's pretty impossible to use the wrong powder just once, at least the way my MEC works. You can't double charge a round unless something really weird happens to the charge bar, which is possible, but the loader would notice it's harder to crimp.
I think it's a mystery with a solution that's going to be impossible to solve. Just one of those things that science, not gun guys MIGHT solve, if any were interested in studying it.
