Drought can do that but it usually tkes a little longer, unless it was so severe the bush wilted. Other wise, I would look for bad root damage from digging, gophers, grubs, etc. Some diseases can also do that, but we would need to know what kind of bush it was and how it was treated to make a stab at diagnosing that. If you join GoY (it's free) you can reply, and answer questions, and engage in discussions about it!