2-piper's theory of a possible cook-off would be easy enough to check if you kept your fired shells. One empty would not have a firing pin indentation if it went off without a hammer dropping. I'm thinking it was more likely a worn sear due to heavy use or improper hardening. It could have been on the verge of doubling and the extra jostling from opening it let it off.
If you send it back to the maker rather than a good independant gunsmith, I think you're less likely to hear them admit the true cause if it's due to defective parts. The independant gunsmith would have no vested interest in not having such information out there to possibly be used in a future lawsuit. Hope you'll follow up and let us know what you find out.
