I thought AR lowers were supposed to be these low stress items that you could almost make out of cheesecake.
I know people claim that the AR lower is very low stress but it does take some stress. Recoil affects it (yes, 5.56 has recoil, even though for the shooter it is very light), plus you got the whole BCG slamming back into the reciever extension.
That said, I think the aluminum they were casting into AR lowers was very low low low quality (like think screen door quality) and such so their poorly cast pieces of *expletive deleted* that it broke apart on the second shot.