Those bad actions don't "wear out", or show cracks, or "reach a breaking point", and the number of rounds fired or the age of the action is irrelevant. They are brittle, like a fine china plate. If you drop the china plate, it breaks, no matter how many meals have been eaten from it or how old it is.
And pressure also has little to do with it. Those actions stood proof, and some were proven more than once. But a sharp pressure blow shattered them. As an example, one that let go was fired with what the owner called his "rat shooting load", 9 grains of Bullseye and a round ball. Not an overload, not high pressure, but a very sharp blow.
And I know about that, as I struck one of those receivers with a 1 pound hammer and it broke in three pieces (I hit it in the right receiver rail, and the rail broke into two pieces, while the both ends and the left rail remained together.) A friend and I had been shooting that rifle the day before; he had put a couple of thousand rounds of GI ball ($.02 a round in those days) through it. (He was sending it back to DCM for a replacement, and condition didn't matter.)