Corrosion of the outer surface
Fretting corrosion of the outer surface of the bearing is often the result of a loose fitting bearing.

A mis-application (wrong part number) is possible, but more likely a worn hub or spindle is to blame.

Overheating
Burnt rollers. Overheated bearing steel develops very characteristic patterns. The colors reflect the temperatures. A strawberry brown indicates a 300°F temperature and deep purple means the heat went over 700°F.

The cause of overheating is lack of lubrication, over tightening during bearing adjustment, or overloading the carrying capability of the bearing.

Spalling
Fatigued metal. Spalling occurs as fatigued metal lifts off of the surface. The example pictured here is extreme, but not wholly unlikely. You may find surfaces where only small patches have begun to flake and disintegrate.

Spalling will occur at the normal end of bearing life. If it occurs early, the likely causes are lack of sufficient lubrication and excessive loading.

Arcing
Electrical damage. Electric arc welding on the engine or chassis without proper grounding can destroy the wheel bearings. The electrcial charge passes through the bearing assembly and lifts off the bearing surface.

There are some other electrical charges that will cause this problem (besides lightning), but I don't know what they are. We need to talk to Steve or Bill.