Something they ate (naturally a given).
Parasite/bacteria imbalance.. Bacteria is naturally good for breaking down.. Its all a matter of good:bad bacteria culture.
They can get totally pissed off at the owners, start to poop smear etc... Any number things can do this - different people in the house, they dont like the color of your cloths or a scent and the list goes on lol... THey have their way of letting us know. ALSO a territorial response and males in mating season are well known for showing their dominance in this fashion..
And then there is the "normal poop place" in which case like a puppy peeing on a rug.. YOu have to reprogram this behavior.. And they CAN be very stubborn about it.
And then are are other various health concerns..