Your shrubs could have Verticillium Wilt, which is a disease caused by fungal spores in the soil. Unfortunately, it is very difficult to get rid of. The general recommendation is to take out the plant once it succumbs, dig out as much of the soil as is feasible, replace it with clean soil, and replant with a species not susceptible to the disease (not another Rhododendron).
Your shrubs could have Verticillium Wilt, which is a disease caused by fungal spores in the soil. Unfortunately, it is very difficult to get rid of. The general recommendation is to take out the plant once it succumbs, dig out as much of the soil as is feasible, replace it with clean soil, and replant with a species not susceptible to the disease (not another Rhododendron).