Well, I don't know too much about the reasons why...but i could guess. Dogs have three very strong senses. 1) smell-the can smell the barbeque three blocks over. 2) Hearing-
one example for me is my dogs are kept in my yard and if they hear my bike from the next block over they'll start barking. 3) Taste- here's where i believe the problem lies. If the dog smells something and can't decern it from the smell, the next thing it's going to try is to taste it. Now, I could be comletely wrong, but maybe one of it's senses is weaker and so it uses it's taste to decifer who's "marked the spot". It's just a thought, i'm no professional, but it might be worth looking in to.

------------------
Every problem has a gift for you in its hands.
-Richard Bach