There are many possible reasons, including improper tapping at the factory or some other defect.
I had the barrel blow off a Ruger "unmentionable", Ruger didn't want to talk about it, they mailed me a shipping label, and sent a brand new one to my FFL dealer. They could not tell me what happened. I think it was a manufacturing defect.
There could be a batch of these rifles that had loose nipple threads, a bad threading lube was used, who knows. This is the nature of the beast with mass produced firearms.