Wow. I have seen a handful of cases of pistols firing out of battery, but never a catastrophic failure like that! Typically, the cartridge case will bulge a bit just forward of the head. There appears to be some type of corrosion --or something-- inside the case. Not sure what that might be. The chamber seems to be bulged... very unusual with a load of the correct pressure. I agree that we might be looking at a big time overcharge of a handload, or perhaps a totally wrong cartridge loaded into the gun, e.g. a 9x21 into a 9mm or a 10mm into a .40 S&W. That theoretically could leave enough unsupported case wall area to explode like that.
I have had a pistol fire out of battery exactly once in 40+ years that I have been shooting. I bought a used Browning Hi-Power that turned out to have had a trigger job done by Jackass Joe in his basement. About one time in five, the hammer would ride the slide home and pretty much slam-fire. The case bulged but did not explode. I got rid of that one in a hurry, needless to say!