I could be a borer or drought damage.

 Drought can cause entire branches or entire plants to die. We have had several years of drought here and we observe that if rhododendrons and azaleas are not watered during a drought some plants will die, but others will just have one section of the plant die. It seems to be the plants way to conserve what little moisture it has. Prolonged drought weakens plants and often results in the appearance of fungal cankers on the branches of older azaleas. Look for branches that wilt in hot, dry weather in late summer and be sure to water azaleas if drought drags on more than a few weeks. Prune out the affected branches to stop the spread of fungal canker diseases.

 If a Rhododendron Stem Borer, Oberea myops, or Rhododendron Borer, Synanthedon rhododendri, is in a branch, the entire portions of a plant beyond (away from the roots) will wilt and die. Borers only affect the portion of the plant away from the roots from the borer. If the borer is in the main trunk, then the entire plant will wilt and die. The plant can be saved by cutting off the area with the borer and letting the plant regenerate from the roots. There are no conventional insecticides that will kill stem borer larvae once they are inside the branches. The best control option for homeowners with only a few plants is to prune out and destroy wilting branches in early spring or late summer.

Thanks.