Unfortunately, I believe your viburnum has a bacterial disease called Verticillium Wilt. This is from a bacteria that lives in the soil, which can stay dormant for years.  It can be a fatal disease that effects the vascular system of the plant resulting in dieback of one branch at a time or half the shrub at once. This disease typically appears in July in our growth zone. 
Other symptoms include:

• Leaves will turn yellow and brown around the edges before the become wilted
• Discoloration on the bark of infected branches

To treat prune all the effected branches back to at least 2' behind the discoloration. Make sure you sterilize your pruners before and after using them  with alcohol or bleach, n order to prevent spreading the disease to other plants.Drench with a fungicide that contains prothioconazole. 

There is a possibility the shrub will die. If it does end up dying do not plant anything else there until you have colorized the soil to kill the bacteria.

The best defense for preventing this from happening is making sure the plants have the appropriate environment. They are getting the light they need and enough air circulation. Irrigation after the plant is established should only be during dry periods.

I hope this answers your question.