Just dang. What causes such a massive malfunction?
In GI guns, usually the culprit is an “out of battery” discharge”. This happens for perhaps two reasons-a primer that is not seated flush or below flush in the case, or a broken firing pin which protrudes forward of the bolt face (normally, the M1/M1A/M14 FP is retracted by the rear bridge and is freed only at the point of 100% bolt rotation).
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In either case, the cartridge is fired before the bolt is locked and since the cartridge is contained partially by the chamber, the head ( rear end of the case) ruptures, releasing 50k PSI into the action and surrounding wood. Often the bolt is broken, the stock is shattered and the box mag (M1A/14) is destroyed. The shooter often endures blast wounds and fragments.
As far as the action failing under a normally seated and locked condition- I do not think any GI M1 or 14 has ever been documented to fail. In fact, MG Hatcher of the US Army ord dept ground off the lugs on a an M1 until only 1/10 of an inch remained, both on the bolt and in the receiver (left lug, right lug is integral with the right rail), and the gun still held together with repeated firings of both M2 ball (50k psi) and proof(70k psi) rounds. The M1/14 actions are among the toughest of all actions, period.