Although this can happen out of the blue and for no good reason and a multitude of reasons, the biggest culprit is disturbance. If your cookie is just crusted over and you tilt or jiggle it, it can break the crust and or cause this unsightly ripple on your surface. Ive even heard of ripples coming from kids running in the house while cookies were crusting over. The short Workshop time means your cookies are EXTRA prone to this. For this reason, I will give several reminders to always try to keep your cookie as flat as possible during transfers in and out of the dehydrator. If drying cookies at home, its best to put them in an undisturbed location with the fan on.