I have a 3 year old female schipperke. she has been trained at dog school. In the last 6 months she has become aggressive towards other dogs but only while she is tied up, backed into a corner or on a leash, she snaps, growls and would bite other dogs if I didnt intervene. She is not aggressive to humans and is generally a submissive dog. Why would she be doing this I think its fear but Im not sure, and what can I do to stop this? she is not aggressive to dogs unleashed but she sometimes looks intimidated and shies away. This is new behaviour and she has never been attacked or abused by dogs or other people. Please help me!