As an "expert" on blown-up firearms (he testified at many trials over the decades) once pointed out, much of the time it is impossible to tell exactly what caused a firearm to come apart upon firing, because most of the evidence (especially the ammo) is all busted up. Was it the firearm or the ammo, or something foreign introduced into the "system"? Much of the time it's hard to tell.
FWIW I spent several years doing failure investigations, and while one might be cautious about saying dogmatically "exactly what caused" a failure there are features of different modes of failure which will be apparent if not obvious. For example, a brittle receiver will fail in a different manner from a tougher receiver. You can pretty much tell at a glance whether a receiver failed in a brittle manner, though of course you'd scrutinise closely to eliminate other possibilities, and then probably say something wooly like "upon initial macro examination the fracture morphology exhibited features consistent with brittle overload failure" to cover your butt.
Without getting overly technical about it that was the point about the low-number rifles: that when they did let go it was in an obviously brittle manner, flying to pieces rather than bulging and distorting but basically staying in one piece as a tougher action might. Failing in a brittle manner is of course a Very Bad Thing, as the bits fly free and act as shrapnel. The receiver is said to have a low "work of fracture" so the applied energy isn't absorbed in actually distorting the metal and forming the fracture but instead is translated into driving this shrapnel and hot gas into an expanding circle of unpleasant consequences.
Now the other thing of course is that a brittle receiver may be perfectly fine under ordinary conditions but if you couple a brittle receiver with some pressure excursion you don't have the toughness there to cope, and so when coupled with say a case head failure the reult won't just be some hot gas vented out of the action but rather a kaboom.
That said though the actual incidence of failure of these receivers is very low - almost vanishingly low really - though of course that would be no consolation to those few blokes who experienced it. It would appear that there were some small percentage off-spec, and this was discovered more as a result of the way they coped with case-head failures and other out-of-the-ordinary conditions than anything.
