The mature one that is dying

The one planted 2 years ago. It bloomed this year, but has not really thrived

It would be helpful to know the name of the particular varieties of rhododendrons and diagnosis by photograph is not terribly reliable. However... These rhododendrons are awfully small for being 15+ years in the ground. Have they been regularly pruned? In general, the sudden collapse of an entire rhododendron plant is usually due to phytophthora fungal infection of the roots which clogs the plant's circulatory system preventing the uptake of water and quickly causing death. There is really no cure, but is a sign of poor drainage combined with high soil temperatures. Borers would be very unlikely to effect an entire plant all at once.

Oh - I didn't realize they are small for their age. Before the one, and then the second, started going downhill they were about 6 ft tall. The first one pictured still is doing fine - and is @ 6 ft tall.

No I don't know what kind they are - they were here when we bought the house. The house and Rhodes were first established 1997 - 1998.

The roots are shaded by the deck they are planted next to. They have grown up enough that the tops clear the deck and get more sun now. The deck, and these plants are on the north side of the house.

Six feet would be a more reasonable height after 15 years. I was misled by the photo and my assumption that the deck was not as high as it apparently is. The suddeness of the decline remains typical of phytophthora. Without any clue as to where you are located, it's hard to make a recommendation for a better analysis of your problem - if you are in a rhododendron growing area such as the PNW there are any number of experienced gardeners who could help. Your County Extension Service and or state agriculture department are other resources.