I've been thining about this and I've come to the conclusion that it was a natural occurence.
I think it comes down to the fact dogs just want to fight over an object and that humans are too lazy to go running after it. So humans throw it away just to give themselves breathing space and dogs instinctively think the human is hot on their tails to retrieve it.
P.S. If that doesn't generate enough conversation can anyone answer if polar bears hibernate in the colder months?