IDK - he might associate it with the odor of feces in the tray, which many dogs like to eat, play with and wallow in given the chance. Dogs seem to go through certain stages where they are obsessed with vile things humans detest. That stuff might make him sick or block his intestines so better fence it off from him.
__________________  Jeanie and Tibbe One must do the best one can. You may get some marks for a very imperfect answer: you will certainly get none for leaving the question alone. C. S. Lewis |