Wheel bearings are designed for longevity. Their life expectancy, based on metal fatigue, can usually be calculated if general operating conditions are known. Bearing failures not caused by normal material fatigue are called permanent failures. The causes can range from improper lubrication or incorrect mounting, to poor condition of the shaft housing or bearing surfaces.
Inspect the bearings for these improper wear conditions, and for:
If you find damage. The bearings should be replaced.