[asphaltgambler]

What you are seeing is 'brinneling' of the wearing surfaces. The hardened part of the rollers as well as the race has worn past and into the base metal. The result could be wrong type of oil, contaminates in the oil, or the oil has overheated to the point where it looses its lubricity properties and breaks down.

And / or overloading / over stressing those components. Lastly, improper installation where there was too much pre-loading on the bearings by not correctly shimming causing overheating under load. It can also be a combination of these factors