I don’t think the liquid nitrogen changed much. What happened was going to happen no matter what because of the properties of cast iron. The only thing the liquid nitrogen added was speed of freezing and the cloud of vapor, making it look more dramatic. But the iron was going to fail the same way, regardless of how slowly or quickly it froze.
The key to “when” it would fail is the amount of solid ice that formed. Each 1.0 ml of water that became 1.1 ml of ice added to the pressure in the container. Once the critical volume of ice was formed, the pressure exceeded the strength of the iron.
From the fractured edge, it looks like gray iron, which has lower tensile strength than ductile iron.
When the iron failed, you can see that it shattered at the point of highest stress, which is where the cap’s curvature is sharpest. And it was explosive because of the brittleness of grey iron. When the first stress fracture formed, the fracture probably traveled around the cap at the speed of sound in iron, with no elasticity to slow the spread. So the whole dome of the end cap fractured all at once and blew off as shrapnel, and the remaining unfrozen water under pressure acted as a rocket engine to blast the pipe away.