Detroit 60 12.7 ate the camshaft and the corresponding rocker arm lobe. What could cause this?

So, the engine developed a kind of rattle above 1400rpm. After confirming that there was nothing wrong on the exterior of the engine (belts, tensioners, pulleys), I dedicated today to inspecting the overhead. To my astonishment, I discovered what is shown in the attached photos.
The cam lobe and one roller had literally "eaten" into each other. I didn't notice any other symptoms, no warning signs, excellent fuel economy, no smoke, no oil consumption, etc. The engine has only about 400k miles on it.
I am quite worried about why and how this occurred. The repair is beyond my expertise, and I will probably take it to a local repair shop, but I would greatly appreciate any insights on what might have led to this issue.
I am diligent about maintenance, performing oil changes every 15k miles, using Rotella T6, etc. Any thoughts on this?