How does a water pump go bad?

Typically the main bearing goes bad and causes the fluid to leak out. Antifreeze has a lubricant in it which helps to lubricate this bearing. If only water is used in the radiator, this would shorten the life of the bearing. Otherwise, just normal usage would cause it to fail at some time. Premature failure can also be caused by having the drive belt too tight on it. A serpentine belt with automatic tension helps to prevent this problem.