Unlike their canine cousins, cats are not pack animals. Although a cat will often tolerate the presence of other cats -- and dogs -- in their household, all this company can be stressful. And this stress can lead to litter box problems.
When grouped together, cats will establish a social hierarchy that includes multiple territories. For example, while one cat may be dominant in the area where food and water are served, another may be dominant in litter box territory. These complicated relationships require cats to create routes that don't cross the path of the dominant cat. So when the cat who rules the litter box area decides to take a lengthy nap smack-dab in the path to the litter box, it can cause subordinate cats to relieve themselves on other surfaces throughout the home.
You can help keep the peace by placing multiple litter boxes for easy access. Put a bell on the collar of the cat who dominates the litter box area so other cats can hear her coming, and consider installing some vertical resting spots to ease tensions. Elevated shelves, cat "trees" and cat walkways can reduce testy interactions between cats [source: Litter Box Guru].