Search for "Phytophthora root rot" and see if this is a possibility.

It could also be rhododendron borer, affecting only certain branches.

Bark split - may happen in harsh early fall freezes or late spring hard frosts, when the plant is not dormant, symptoms may not reveal themselves until later.

Even drought - sometimes a rhododendron will respond to lack of water by a section rather than the entire plant dying. Was the root ball opened up when the plant was installed so that the roots could reach out and expand into surrounding soil?

Pop the plant out of the soil and take a look at the root ball. That's what I do if I see no signs of obvious trouble above ground. Maybe it's too compact because it wasn't ruffled up a bit before planting and now it's not absorbing water and expanding out to the existing soil, like Morz8 says.
Mike

Almost 20 years ago I had some large leaf Rhodys that just didn't seem to like their locations. After a few years of sickly appearance & failure to thrive, I became so fed up that I literally ripped them out, roots and all, and "tossed" (not planted) the bare root balls in an area next to an outbuilding with the intention of "disposing" if them later. Well, I got lazy and left them there.
By the end of that summer, they both had new beautiful leaves growing from the base of the stem, and 4 years later I had the most gorgeous 10 feet tall Rhodies. It just came down to the original location not giving the plants what they needed, and accidentally finding the ideal location. Since 8 years is plenty of time, I say move it to a new location, maybe with more sun (my original site was very shady, the "accidental site was significantly sunnier). Mine thrived after that!

Thanks all. Not sure what the problem is, but I decided - based on these responses - to move the plant. I was reminded by reading your answers how rhodos do need soil that drains well. It was in an entirely very deep shade location, and possibly a bit too murky. So, I moved it into a dryer, much lighter degree of shade location, Maybe a couple hours of direct sun, too.

I found the tag from it. It's a Catawbiense Album.

Thanks all.

Catawbiens Album is a tough plant. It will take full sun but likes part shade.

All rhododendrons need some sun and excellent drainage.

This is more likely borers or drought damage. Bark split and phytophthera will kill the entire plant. Borers and drought damage just affect individual branches.

Here is a link that might be useful: How to care for rhododendrons