I've never noticed that, on beardless irises before, but on the bearded, I usually blame the weather......a sudden warm spell accelerates the growth, and the new "inside " leaf on the fan grows "faster" than the "older" outside leaf can open up, to allow it to emerge.
How is your soil ph ? Beardless like it well to the acid side, and soil that is 'too sweet' (too much lime) will retard bloom, and, even kill the plants.
