This guy had probably been chucking his "hot-loads" through this rifle for years. All the time weakening the rings/chamber and tempting fate. Sounds to me like the thing finally gave up. Hot-loads + old rifle = roulette IMO.
EBUCK
Folks also sometimes forget the effect of ambient temperature on pressure. A max load with safe pressure when it's in the 50's can be too hot when the temp is in the 90's.