I can only see two ways, the cartridge generated a lot more pressure because of wrong or more powder then it should have. The second reason is that the bullet does not move down the bore freely or at all. Any other reasons that would make it have more pressure?
The article I linked to says the arsenals tested these receivers to 75,000 pounds per square inch on the proof test which is considerable to say the least. Because of this I'd bet on the bullet in the bore scenario in most cases that involve high enough pressure to wreck an action especially when coupled with suspect brass of any kind.
It is amazing how much of an overload an action can take all other things considered. I remember an article in a Rifle-Shooter magazine once where the author purposely blew up a Swedish Mauser and I believe an Enfield. One statement I never forgot was "The Mauser 96 may be a piece of junk, but it is a very quality piece of junk". I don't remember how much of a load it took to destroy it but it was way more than any sane person would ever do.
