Actions are bonded to chassis to "bed" them to fill voids and increase accuracy just like customs are bedded. Those screws don't back off unless they're messed with... and both screws at the same time... what are the odds of that? Also not sure how those bolts could shear at the very top of the shroud instead of shearing at the head if this were the case.
I'd put my money on the screws being severely overtightened and they were finally stressed enough from firing to fail. That also explains how they could both fail at the same time.
No matter how robust you make a gun, you can't make it fool proof. Over tighten shit and it's gonna break. Period.