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.
Over time, cat has developed an aversion to texture of litter. Cat was never properly trained to use litterbox. Cat is stressed by change in routine such as new baby, new furniture, new carpet, work schedule changes, vacations, overnight guests or a move. One in every 10 cats will experience a litterbox lapse at some point in their lives. Understanding that sudden change if any kind will upset your feline friend will help you in making the right decisions concerning your pet.