I'm guessing its either some kind of root fungus or maybe they got borers but I haven't heard of either of these being common with viburnums.

that is where root problems begin

ken

My research tells me that viburnums, in general, are susceptible to two stem diseases: verticillium wilt and botryosphaeria dieback. The verticillium fungus can be fatal. It infects roots stressed by drought and spreads upward in the vascular system, resulting in dieback one branch at a time. Botryosphaeria is a less serious problem. It infects stems during wet periods, then spreads and causes dieback once the plant is stressed by factors such as drought. Also, when heavy pruning is done in the summer, sunscald of the leaves is sometimes a temporary problem.

The drought of the last few years was very hard on most viburnums. All my double-file viburnums experienced dieback, but are still living. My Mohawk viburnum was unphased, but it was also incorporated into a perennial bed, so it was watered regularly during the drought and that's probably why it suffered less.