Hi, Well there's no way of knowing without examining the plants in person. Bearded iris can be affected by the iris borer which you can find it you dig around and find rhizomes with entry holes in them and or any signs that they are being eaten. Bearded iris also do not like to stand in wet, poorly drained soil so if the plants are in really mucky soil, it could be that the roots are rotting. I just can't be sure without seeing the plants. Check the soil and the roots to see what you can find that might give the culprit away.