Moults can cause some rather serious digestive complications. Hair balls cannot be regurgitated in the good-old cat fashion as rabbits lack a strong enough gag reflex. To keep their systems in good working order make sure they are eating plenty of hay and drinking plenty of water. If your bunny doesn't appear to be drinking very much try putting thin slices of apple into their water, they will drink the water to get a better grip on the apple (:3