Shirley L. wrote in with a quick comment that many other cat caregivers have made over the years. Shirley writes about her cat, Tinker:
Shirley, we assume you're asking why this happened and what you can do to help prevent Tinker from soiling outside her litter box in the future. In most cases, there isn't a simple answer. There can actually be several causes that, when combined, make Tinker choose not to use the litter box. Many times these causes are related to change. Cats dislike change in general, but they really hate it when their eating, sleeping, or litter areas are disturbed in some way.
Have you recently changed anything about the litter or the box? Switching litters, moving the location of the box, buying a new litter box, cleaning the litter box area with a smelly cleaning product, increasing activity around the litter box area; any of these things can contribute to a distaste for using the box. Cats are much more sensitive to new sights, sounds, and smells than humans are. Visit the litter box area and imagine what Tinker experiences. You may learn quickly what's bothering her.
Are there enough litter boxes for your cats? Is there competition for the boxes? You should always have one more box than you have cats and the boxes should not be visible from one another. Cats use a sort of timeshare method of getting along. They like to avoid confrontation when they can. You can help by making sure Tinker doesn't feel cornered when using her preferred box.
Which brings up types of litter boxes. You'll want to choose one that's open and easy to get in and out of. Covered boxes are no good. See this post for more detailed info on choosing a box and litter.
You also need to make sure that Tinker's box is kept clean. If you're using clumping litter, you can remove deposits twice a day and do a thorough cleaning once a week. If Tinker thinks the litter is too soiled to use, she'll definitely find somewhere else to go.
There's also the possibility that Tinker feels threatened by other cats, either in the household or outside. Interloper cats will often spray outside where Tinker will be able to smell it. That smell will definitely make her feel that her territory is threatened. As a response, she may choose to leave her stink bomb out in the open to mark her territory. See this post for more on making Tinker feel secure.
Last but not least, we mustn't forget that Tinker may be ailing in some way. If she's a senior kitty, she may have joint pain and find it difficult to get in and out of her litter box. She could also have an ailment that makes her less able to control her bladder and bowel movements. It would definitely be a good time to take Tinker to the vet for a checkup just to make sure there's nothing wrong.
It's an extremely complicated issue, Shirley, but it's one that can almost always be solved. We know you must love Tinker very much, so don't give up on her. We're sure you can sort it out.