The proper adjustment of the two bearings in the hub assembly is one of the most important aspects of the installation process.Excessive tightening of the adjustment nut, otherwise known as excessive preload, can cause the type of rapid deep spalling shown in these pictures. This deep spalling occurs when the direct contact between rollers and raceway caused by excessive preload super heats the bearing, softening the material, and allowing chunks to be torn away from the raceway and rollers during operation.

On the other side of the spectrum, loosening the adjustment nut too much will cause excessive end play in the bearings, allowing a rocking motion that disrupts the even distribution of force along the rollers. This, too, can cause bearing failure resulting in damage similar to that in this picture. As you can see, the damage is restricted to one section of the raceway that underwent extreme stress due to excessive end play.
The key then is to hit the sweet spot between too much end play and not enough. For Timken bearings, this means achieving an end play between .001 inches and .005 inches. To help, Timken has established an easily measurable three step adjustment procedure.