What do you call this?

[Sam1911]

The High Point C9 is a blow back operated pistol, NOT a locked breach gun.

The reloads you were using were for locked breach guns. When the gun fired the slide started to move back before case pressure was low enough and there was a case rupture.

Nope. Not possible.

There are no "blowback pistol" 9mm loads and "locked breech" 9mm loads.

HiPoint makes a gun to shoot 9mm Luger/Parabellum ammo, period. As the OP said, they even rate it for +P ammo.

While there are several possible problems which could have caused the blowout using "for locked breech only" ammo isn't one of them.

 

[Sam1911]

Right, but the inertial mass of the slide is what keeps it locked during firing, not the spring.

Again correct. Regardless of how worn out the spring might be, or how brand new, when the gun is in battery the spring is putting very little pressure on the slide. It is close(est) to full extension at that point and cannot be expected to resist the pressure of the cartridge firing.