I was down on the Blue Ridge Parkway this spring when many of the rhododendrons and azaleas were in bloom. I didn't see any R. maximum blooming since it blooms much later than than when I was there.

Your native rosebay rhododendron is Rhododendron maximum. It is occasionally severely damaged from deer browse. It needs a moist location with good drainage. We have had some serious droughts in the last couple years and had extremely hot weather this past winter which is very unusual. It is sensitive to many problems:

soil compaction - intolerant
pollution - intolerant
deicing salts - very intolerant
urban heat islands - intolerant
drought - intolerant
seasonal flooding - tolerant

In addition, it gets stem canker, bud and twig blights, crown rot, root rot, azalea stem borer, azalea leaf miner, and suffers severely from winter desiccation. In times of increased stress, disease problems are much worse. For example, drought causes more stem cankers. If your winter was dry and hot, that would have been a huge stress.

Another problem is that invasive exotic plants increase competition and make the above problems worse. I don't know for sure what you are seeing, but it is probably a combination of a couple of the above problems.