Friction is a chief cause of bearing failure. Insufficient lubricant reaches the gap between the shaft and the inner bearing surface. The end result is the assembly heats up significantly. The heat causes stress cracks and even melting to develop, and the bearing fails. For ball and roller bearings, the same effect takes place, except that friction causes the balls or rollers to fail.
Stress fatigue is another common cause of bearing failure. If you bend a piece of wire back and forth continuously, the wire eventually will break. The same sort of cyclic stress fatigue happens in a bearing. If the bearing undergoes vibration, stress cracks start to form. These cracks get deeper and deeper, and eventually the bearing fails.
If you keep hammering a piece of metal, the metal eventually will start to crack. The same sort of hammering stress can occur in a bearing. If the shaft is too small for the bearing, it will start to rattle. The rattling causes stress cracks to form. These cracks travel through the bearing, and the end result is eventual bearing failure. For tapered roller wheel bearings in a vehicle, this is significant. If the main axle nut loosens up, the rotor will cause the bearing to chatter. This chattering will eventually hammer the bearing into failure.
Dirt and grit in the lubricant can cause bearing failure. Dirt gets in between the shaft and the bearing surface. The dirt starts to grind away the shaft and the bearing, causing heat, and the shaft also becomes under-size. When the shaft becomes under-size, chattering sets in. A snowball effect takes place, and the end result is bearing failure.