I don't know why some does do it. Once could be sort of accidental, but I tend to think (unsubstantiated opinion here) that if a doe pees on her kits habitually that it is an act of rejection or aggression. About all you can do at this point is what you are doing: changing the bedding, warming them up and being vigilant. You could remove the nest box to the house and just return it for twice daily feedings until the kits have their eyes open and are mobile, but if he mothering instincts are weak this is not likely to strengthen them. Cross you fingers that it was a one-time goof and lay in some extra bedding (dryer lint, feathers from a pillow, teased cotton balls or batting, even shredded newspaper) so you have it available in case of need.