The two most common causes of sudden dieback are both fungal infections. Various strains of phytophthora cause root rot. This disease is more common in pots, but overwatering, high soil temperatures and poor drainage can lead to it in the landscape as well.

Botryosphaeria effects branches and stems and can appear suddenly - often as a result of droughty conditions. This is probably more common than phytophthora in the landscape.

Unfortunately, there are really no good treatment options once an infection has appeared. Pruning back to healthy green wood, cleaning up and disposing of any fallen leaves, and making sure growing conditions are the best you can provide are the best things you can do.

There's more good information available at rhodyman's website.