80% of the time inappropriate elimination problems are medical, not behavioral. It is important for her to see your veterinarian. Going on clothes can be trying to choose something soft to go on as pain can be associated with the box. She may think it'll hurt less if she goes on something soft. She may have urinated on the items first, and where she can urinate, she feels it's appropriate to defecate, so urine should be checked as well as abdominal colon palpation to start.
The sooner it's checked out, the better success with treatment. Your veterinarian will work hard to find the answer to your frustration with, "why???" Best of luck!
Dr. Mathis gave a great answer. I just wanted to add a little. There are many possible causes for defecation out of the box including parasites, intestinal problems (leading to soft stool and urgency), anal sac problems, behavioral, unappealing litter box.
Remember to bring a fecal sample to the veterinarian to be tested for parasites. Have your veterinarian check your cat's anal sacs. Watch the stool for any change in form, mucus, or blood and let your vet know what you see.
Always keep the litter box very clean, scoop daily and change the litter entirely weekly. If there is a cover on the box, remove it. Do not keep the litter box where something could startle your cat (laundry room with washing machine that kicks on suddenly or in front of a vent). Use a soft scoopable dust and scent free litter.
Judy Karnia, DVM
Scottsdale Cat Clinic