That sounds like what happened to my Pink Poodle echinaceas.
One plant survived but it was weakened and last year succumbed to aster wilt.

Looks like its a goner. Pull a stem up and see if you have aphids at the crown. I have lost a ech that way. You can also get rot at the crown if you keep mulch too close as well... Though it doesn't appear that would be an issue in your case.

I have had my ech pink poodle for many years now and it doesn't seem to require anything different then the rest.

Who knows how I got on this page--ah, the mysteries of the technological world and little boxes.
What I was interested in reading was how well pink poodle did since I've just purchased one, but it seems, like everything else, nothing is for sure.

I've had my Pink Poodle for a couple years now and I love it, but I noticed it is now starting to look similar to the above photo. No yellow leaves at all, and flowered beautifully this year, didn't notice deformed flowers out of the ordinary (it never blooms perfectly double, it's always a bit off). Never had problems w/rotting in years previous, so I doubt that's it. I guess I should dig it up to see if anything is making a meal out of the roots or crown. Sigh...