I've shot some overloaded BP in muzzle loaders. I don't think BP works like smokeless at all, in that the added quantity doesn't raise pressures significantly over smaller quantities. It just creates a longer push.
I could be wrong, but I've heard of double-charges before from friends. More recoil, a big flame out of the end of the barrel, and seeming higher velocity for the projectile, but no danger of boom. Still, you gotta follow the rules for compressing the powder, etc. That's why I think it was operator error, most likely. You have to do something wrong IME to create gun-destroying pressure in muzzle loaders. There is a possibility of flawed steel in the barrel, but this is very unlikely.
