Looks to me like out of battery firing. The breech was not fully closed, but the gun fired anyway (that should be tested). The inward bending of the rim indicates that it was not against the chamber edge when the round fired.
Why did the breech not close? Could have been a buildup of dirt/grease/wax in the chamber, an oversize bullet, or some other reason. Anyway, there was enough support that the firing pin fired the primer, but the unsupported part of the case blew out. Which way the gases went, I don't know for sure, but probably upward since the OP doesn't mention damage to the magazine or grips which would usually result if the gas escape was downward.
IMHO, there was no defect in the cartridge unless the bullet was oversize causing the failure of the slide to go into battery. As to why only that round? I don't know. Something might have kept the slide from going fully closed - the thumb, a part of the clothing, I don't know.