Very happy to hear you're not seriously injured. I've had two barrel blow ups--nothing quite so serious--and I can appreciate the Level 10 pucker factor.
As for what happened, it may never been known for certain, but if you or a friend has a bullet puller, an interesting experiment would be to pull those remaining rounds and look at the charge weights. We may not have a clue as to what powder is being used, but it doesn't matter really if you see a wide variation in the charge weight.
Recently Shadow posted the results of a pulldown he did from a small ammo manufacturer, and found two rounds with a WILD variation--as in near-squib sub-grain charge in one, extra biggie in the other.
So the explanation could be that simple: pushing maximum loads and then having crappy process control could easily lead to a blown gun.
My opinion is also that these guys who are selling ammo probably in very few if any cases have actually taken the time to set up a lab where they can measure pressure. They're just guessing. That's my opinion. I don't really care who they are--if you don't measure both your process (charge weight being an important part of it) and your pressures, then you're flat-out guessing at what you're selling.