Hi Gary

Sorry haven't been on puter lately. You already got several good answers and reasons. It is my belief that it generally starts in or with a diet deficiency ? and then becomes a learned behaviour once the animal in question gets a good or bad, from caregiver, acknowledgment. Then others, for the same reasons, can and will kick in and do the same.
Is there anyway you could "isolate" the cat litter box so your pups do not have access to it ? It is my understanding also, for reasons known only to the offender/s, that cat scat is highly desired by dogs ? (eewww inserted here)
Good luck and let us know what you come up with ? I have never heard it is harmful, just disgusting to humans.
BUT, in their natural origins or in the wild, other's scat is highly nutritious, again....eewwwww.
Judy