Owner changes location of litterbox. Owner switches to deodorized litter. New litter box. Owner cleaned litterbox with too harsh a cleaning product. Location of litterbox is not private enough.
Home is too large for one litterbox.
Cat is kept from litterbox by another animal. Too many cats and not enough litterboxes . Too many cats and not enough territory. Unneutered male is marking territory. Unspayed female is in heat and advertising for suitors.