The cat is suffering from a medical problem. The cat may be constipated. Litterbox is not as clean as the cat wants it to be. Owner changes brand of litter abruptly. 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.