answer:I had an Australian shepherd who was raised with a cat. They were together from six weeks old. They were best friends. We eventually brought two new kittens, six week old siblings, into the family. To the dog, the kittens were no big deal. He didn’t seem to care or notice them. Of course I took time with both kittens to introduce them to our dog. One became fast friends with him. The other would stalk and hiss at him years later. Saying all that, I think it is one part that dogs are territorial and protective of their domain. Another part is that perhaps your dog sensed the strange cats’ unease and automatically went into defense mode.