Were the cookies extra thin and long, or soft to start, and/or was it extra humid? I've had this happen a few times, especially on big-gish, long rectangular shapes. My theory is that, as the icing hardens, the cookie is softening and buckling/bending, so the icing doesn't get a chance to dry on a completely flat surface - and thus cracks or ripples as the cookie bends and the icing starts setting up. I've pretty much eliminated this problem by speed-drying these risky shapes at a slightly higher temp than I normally use in my dehydrator - i.e., closer to 105F than 95F. Hope this solves your problem as well!