Short answer - nothing lasts forever. I cracked a 1911 frame in a non critical area putting 35,000 rounds through it one season many, many moons ago. K frame revolver forcing cones are notoriusly fragile with a diet of light bullet/high velocity loads.
Longer take - as most everyone knows the 9mm is a high pressure round; could be a slight weakness in the manufacturing of that particular barrel/billet that eventually said "I quit"; a lot of intangibles as to fit, finish, and stress where the way parts fit together and wear in freakish manner that may only show up in extremely isolated specimens. Luckily it didn't 'let go' and cause further damage or injury and was caught in time.
I've often marveled at the forces and pressures involved that we take for granted just scant inches from our face and hands in handguns and especially in rifles and their cartridges. Having seen graphic results of what can happen when such does fail, I'm reminded that given what we're working with that 'accidents' are usually not one thing but the stacking or confluences of several contributing factors, many of which line up as a result of neglect or carelessness because we've "gotten away with it for years".
Your photos are a caution to us all to not forsake the regular careful examination of these contrivances, especially those we tend to use a lot and can too often take for granted.
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