What makes most diodes blow is when owners try to charge a dead battery with the alternator when driving. Alternators are designed to maintain a charge only and not recharge from scratch. As stated above look for excess current draw from whatever source.

 

The above situation is called a "Load Dump Transient" that can easily hit 400V with enough power to blow just about any electronic component.

The amp rating on newer small frame alternators is only an intermittent rating. The older alternators were often rated continuous amps. Overloading newer alternators is quite common which results in burned out diodes. They just do not have the heatsinks necessary to do the job.