You might want to have the vet look at your kitty and also have a parasite check done on the poop.

One possible cause can also be constipation. One of our hospital's cats has problems with that if she doesn't get her cat laxitive she'll poop in her kitty bed. My cat Jack will poop next to the sandbox when he's constipated really bad, but then again he's been diagnosed with mega colon also, so I know it's a medical problem with him.

Either way a good check up with the vet is a great start to rule out medical problems first, then work on any behavior problems if everything checks out ok, your vet should be able to help you with that also.

Good luck