Your description is confusing. How can the part that died back last year die back again this year. Maybe it is progressing further. One possibility is borers. They will tunnel in a branch, killing everything beyond where they are tunneling. They can come back year after year.

A more likely possibility is Botryosphaeria dothidea or Botryosphaeria dieback. It causes leaves to turn dull green and then brown and roll and droop. Cankers form on branches which may girdle the branch. This is the most common disease of rhododendron in the landscape. A typical symptom of this fungal disease is scattered dying branches on an otherwise healthy plant. Leaves on infected stems turn brown, then droop and roll inward. These leaves often lay flat against the stem and will remain attached. The pathogen can infect all ages of stem tissue through wounds, pruning cuts, and leaf scars. Heat, drought stress, and winter injury can increase disease incidence. Cankers on branches can gradually grow through the wood until the stem becomes girdled. Diseased wood is reddish brown in appearance. Discolored wood viewed in longitudinal cross section often forms a wedge that points toward the center of the stem, and the pith may be darker brown than the surrounding wood. Sanitation and applying a fungicide such as metalaxyl (Subdue) after pruning my provide some control. Plants should be grown in partial shade, with mulch and kept well watered during dry periods. All dying branches should be promptly pruned out in dry weather and all discolored wood should be removed. Plants should also be protected from rough treatment during maintenance activities to prevent unnecessary wounds.

During periods of drought, the most common damage is Botryosphaeria dieback. It attacks specific branches rather than the whole plant.

Botryosphaeria dieback is opportunistic and attacks plants that are stressed, usually due to drought. It is better to cut off diseased wood and water the plant enough to prevent stress. Not too much so as to create stress.