Hmmm. Glad your buddy wasn't seriously injured. There's the fact that a benchrest shooter was killed a few years ago when doing the same trick- the cleaning rod pierced her.
Your possible explanation of a kernel of powder piercing the flash hole sounds kind of plausible. One farfetched idea that struck me was could've the striker bounced a little inside the bolt from the impact of the hammering, just enough to detonate the primer?
Another thought: disassemble the bolt and see what's going on inside it. Broken firing pin?
As to what may have caused the jam, I'm clueless beyond what's been suggested. Dirt/debris?
I would examine that action with a very critical eye before putting it back into service. It experienced enough violence to push the bolt back and out to bust the stock and shed the cutoff, such that stuff in general may be tweaked.
