[Saurreg]

I have read before that animals such as cats and dogs have a sense of awareness that is beyond ours and hence what they detect is not within our capability to comprehend. Perhaps there is some residual aura or potency in the remains of the dead which can only be percieved by such animals and make then uneasy? Orcs included of course.

As for those which do not partake in this gift, perhaps its the reputation of the dead and fear of the unknown that prevents them from entering what they percieve as a "haunted" forbidden ground. Imagine after all, runs wild.