Hah, that sounds enormously funny, especially since my dog does exactly that. At first I thought that he is sick and has some psychological disorders, but later I understood that things work a little differently. They usually do this because they think someone is threatening them, or they can get scared. Especially if you have a small dog that has a phobia of larger people, or other larger animals. Due to the fact that I could not receive some concrete response from veterinarians I decided to do carev courses with reference to veterinary medicine. If you want, look here