Cracked lobe(s) on camshaft of A4 2.5TDI - why would this happen?

[Mildred]

after a few months of intermittent engine knocking, especially bad when cold, my local dealer has recently diagnosed that my A4 2.5TDI (2004, manual) has cracked lobes on the camshaft

1) what would have caused this?
2) do I need to get this repaired at an audi dealer?
3) what other repairs / checks would you recommend I have completed at the same time?

Mildred