The camshaft in a Cummins is a weak link. If you are running a P pump, higher than stock rack travel, and have a tendency to lift your foot quickly off the pedal, you have all ya need to break a cam.
The P pump uses tremendous HP to drive it directly off the camshaft gear. P pumps break em because its like loading up a spring with 25 plus HP drag and letting that spring go when you lift the pedal.
In comparison, a VP44 will use 1/6th the power of a stock P7100 pump and the CP3 uses even less, 1/10th. Thats why you never see broken cams in a VP and CP3 truck. I have never seen it.
If you run a P pump with heavy drag, RPM, and high power, you need a gas carbed steel billet cam for a more bullet proof system. Cast iron or ductile cast iron can only carry so much load.
Still cam failures are rare, but when they occur, you can whipe out an entire engine