Seedlady, I found what I was looking for. If this is, indeed, what you have. Accordian pleating in leaves is caused in some iris species from the leaves becoming obstructed by the enfolding leaves during upward growth. Rapid changes in weather may be the culprit. The leaf continues to grow but can't move up... causing it to fold back and fourth on itself as it grows. When the obstruction is overcome the leaf continues to grow on normally.
On a good note, this is not a disease or caused by an insect. It was most likely a sudden growth spurt from weather changes or perhaps, a high dose of fertilizer. I have seen this in some of my iris clumps. It usually only a few leaves in a clump are effected.