It could very well be the excessive moisture. None of the plants you mentioned enjoy wet feet, and if your soil doesn't drain well enough to handle a very wet season, it could be enough to do them in.

When you replace them, consider improving the drainage in their planting sites.

Actually, drainage definitely was NOT an issue. All of these save one were in a bed at the very highest point of my property at the top of a moderately steep slope with very sandy loam soil.

Are you sure that they are dead? They could just be having a late start ... My Miscanthus are all showing growth but the panicums are only just starting to look anything but dead. Fingers crossed for you.
A.

Mystery Solved! I finally got around to cleaning up these beds this past weekend, hadn't cut last year's old stalks yet.

None of these clumps is totally dead, I did finally begin to see growth on all of them, albeit a couple have just a handful of stalks, nothing like they were.

The cause -- VOLES. Voles had chewed out a lot of the base of some of these grasses, leaving behind a fine debris. I found vole nests in a couple of the clumps. The way the voles did it, I couldn't tell from casual outside observation, but when I began getting into the clumps, it because instantly obvious what the problem was. They seemed to like to destroy the plant at the crown level where roots meet stalks, thus the lack of new growth in destroyed areas.