Thanks for sharing that info. I wonder if somehow those mounts were improperly installed originally? I'm suspicious of the failure generating at the screw holes due to long term corrosion. Pyrodex is some wicked stuff. Perhaps a tiny flaw (maybe induced by the scope mounting??) provided a nice hiding spot for some the residue to hide even though you routinely cleaned the rifle.

Just shooting in the dark here for a reason this happened. You definitely deserve some answers.