My roommate's cat, Zazzy, has pica: she eats things that aren't food. Plastic and metal are particular favourites, but she also will eat dirty laundry (never clean laundry), paper products (books, toilet paper, kleenex, etc), etc. For a few weeks after my roommate adopted her, Zazzy's poops were more non-food-stuffs than actual poop. She previously lived her entire life in a shelter kennel, and that is likely what caused this bizarre compulsive behaviour. And does seem to be a compulsion. She goes into what we call Garburator Mode and can't seem to help herself, especially when she is in the vicinity of her most favourite trigger: chewy plastic wires... and she doesn't just chew them, she eats them.
We have more or less addressed this problem with increased activity (foraging toys, training in cat agility, leashed walks outside) and with limiting her access to her favourite targets. Now, it's very rare to find anything but poop in her poop. We still check, though, just to be aware if she got into anything. We also did some pretty extreme cat-proofing: all our wires are covered in pvc tubes and/or completely duct taped to the wall. The one thing she does not like to eat is tape!