I'd be more inclined to think the guy that was supposed to torque those nuts simply missed them and they left the factory somewhat loose to allow the backer plate to try to rotate every time the brakes were applied. A few brake applications would eventually shear those bolts and allow the plate complete with caliper to rotate and separate your brake line. That's just an opinion but like the guy says: same gun, same compressor, same regulator setting but maybe one dude went to coffee break and forgot where he left off when he returned (needed retraining
) and allowed the axle to leave without checking.
The wheel torque has nothing to do with it as those wheels aren't installed until Doubletree builds a unit. The Lippert tech probably meant the same gun is used for every HUB installation.