In irises, a given section of rhizome only blooms once. After it blooms, it never even produces leaves again. Future blooms will come from new sections of rhizomes, or "increases" as the are commonly called. The old rhizome will (gradually or rapidly, depending on your climate and conditions) rot away. In short, what you were seeing was normal with irises and, as long as the rot wasn't spreading to the increases, nothing to be very concerned about.
