What caused this primer condition?

Interesting. No flattening to speak of, wouldn't suspect excessive pressure.
Breechface eroded around firing pin hole maybe?
 
That's catering; Several probabilities. It could be a pressure sign. The primer hole could be too large. The firing pin could be too long. Is this the only case or is this happening to others as well?
 
Starkherz199 is right. I have never seen creating on handgun rounds of my own, but I have had it when doing load work ups for rifle where the pressures are greater. If they are factory ammo, I would not worry too much, but if reloads, then the question is where are you in the min./max. spectrum, what is your seating depth and maybe how heavy is your crimp. Need to look at what may be causing unusually high pressure.
 
Are they all like that or just this one? Have any pics of the gun, especially the breech face?
 
I would suspect a worn firing pin hole in the breech face. The shoulders of the primer are still rounded, so I don't think it was over pressure.
 
I took the pic from another forum where the same question was asked.
It is a range pickup.

Several folks have indicated it is probably from an open bolt subgun.
 
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