Here's a more simple answer, and we all love simple answers, right?
They made, and make, a lot of guns. For any given group of guns, there will be a percentage that have undetected flaws in the steel which lead to a premature failure. I don't know what that percentage is, but it is very, very small.
The number of guns which burst barrels is similarly very, very small.
Some guns with flaws will never burst - their "flaw vs. pressure" ratio will never be exceeded. Others with lesser flaws will burst as their pressure those flaws see will be greater. I'm sure "frequency of use" also factors into the equation.
So all we are seeing is the normal, natural occurrence of failures attributed to the normal, natural occurrence of manufacturing flaws. The guys who dump heavy loads through a gun are just increasing the likeliness that their particular gun will join the 'burst' crowd.
