Both rhododendrons and Japanese maples need good drainage, if you are on a damp soil and the water table has risen or it has been damper near the surface recently root suffocation could produce death of all or part of the tops.

Otherwise honey fungus (Armillaria) seems possible, as there are often signs of this on old rhododendrons in plantings around here. It attacks the tree or shrub near the soil and kills the top, thereby generating more dead wood for it to decompose - a wood decomposer that doesn't always wait for the wood to be dead from other causes.

I don't think anything has shifted in the water. The rhodies have been there for 30 years. I went out to take a closer look at the trunk to see if I saw any fungi and discovered that this may well have been our doing.

We used to take the dog out with us when we gardened and put him one of those plastic coated cables wrapped around the tree. It wasn't tight, and he didn't seem to pull enough to stress it, so we didn't worry about it. We got tired of taking it in and out and wrapping it the right number of times to control the length, and just left it there, hidden in the bushes. Well, the tree grew. It's biting into the trunk sufficiently that I can't pull it loose. I'm so disgusted with myself - I'm going to go beat myself with dead maple switches now...:-(