Lyn: it's rare but I have got diseased Irises from vendors before and they go to mush like that. And I think mine was a disease because it affected a few other plants close by. It seems to be gone now though.
Also in the Fall when I am cleaning off old leaves from the Rhizome, I just very gently pull. If Something doesn't come off, I either leave it and try to see later if it'll come off more easily or I cut it a short distance from the Rhizome because I found out if I yank them really hard sometimes it introduces rot.
Finally injury's from dogs digging by the Rhizome has caused rot. Most times no, but some just can't take it.
And I know your situation might be none of these but they are just my rot experiences.
