http://www.sniksnak.com/doghealth/aggression4.html
What is redirected aggression and how can it be treated?
Aggression that is directed toward a person or pet that did not initially evoke the aggression is classified as redirected. This is likely to occur when the dog is aroused and a person or other pet intervenes or approaches. Dogs that are highly aroused must be avoided. In some cases a water rifle, air horn, or long leash can be used to safely remove the dog from the situation. If the aggression and arousal does not immediately subside, consider locking your dog in a dark, quiet room, until he settles down and will come out for food or play. Since redirected aggression arises out of other forms of aggression, it is important to identify and treat the initial cause of aggression (e.g. fear, territorial, sibling rivalry, etc.), or to prevent the problem. This can be acomplished by avoiding exposure to the stimulus for aggression or by keeping a leash and head collar or leash and muzzle on your dog when exposure is possible.