Any on or a combination of the above will cause a bearing to become overloaded. When a bearing is overloaded, it generates heat. Heat will cause lubricant to decrease in viscosity, and as it does, it loses the ability to support the load. It will eventually form a varnish residue that will destroy the ability of the grease to lubricate the bearing, when that happens, both moisture and solid particles are able to get into the lubricant. The introduction of moisture and solids into the lubrication is probably the most common cause of bearing failures.