I suspect that the ruptured case was more of a symptom than a cause.
The firing pin retaining pins are tiny, and they're known to fracture, which can lead to an uncaptured firing pin.
On firing, the firing pin supports a portion of the case while chamber pressure is high, preventing it from blowing out. If the firing pin retainer is broken, it might not be up to doing that, possibly leading to a case rupture, escaped gas, and the firing pin being pushed out of the slide.
The relatively small area of the case rupture, the small amount of gas, and relative mass of the firing pin pretty much guarantees it would be a very low velocity event... Still enough to get your attention if it clocks you, though.