I'm not sure, but maybe someone else knows more about this. If I had to guess what would cause this all of a sudden,it would be maybe he's jealous of the other dog? You didn't say how long you had the Westie, he could be marking his scent to show dominance over the other dog, or is Arnold neutered? Sometimes unneutered dogs will spray. Hopefully somebody will come along with some advice for you. Good luck.