Why does diode shorted?

Diodes get damaged when their reverse voltage or forward current ratings are exceeded. It is also possible that a high speed transient could exceed the forward voltage rating before the diode turns on, but this is not normally the case.

In a car, primarily in the alternator, diodes get damaged by excessive current draw, or by connecting batteries backwards when jump starting. One way to get excessive draw is to attempt to rev the source engine while cranking the target engine - that is not a good idea - you should only rev the source engine when you are trying to add some charge to the target battery - you should let the source engine idle when cranking the target engine.