Are you east of the Rockies? Could it be iris borers? That is exactly what people describe.

I have clay soil in a temperate climate with 40+ inches of rainfall each year. My bearded iris rhizomes are about 2/3 exposed (above the ground). I still see some rotting each year. If your iris are buried, that's certainly contributing to the problem. After they bloom, I'd lift them and plant them higher. Maybe you could also amend the bed to improve drainage. If you've been mulching them, remove the mulch as well. Good luck.

Mine did that with iris borers. Immediate plan is to use nematodes, and then make sure you do a good trim and cleanup once cooler temps start in the fall.