Here are a few very informative sites that I found on my search. Some discuss the cause other the cure. Why Conflict Occurs
Conflicts between household dogs develop for a wide variety of reasons. Conflicts may occur if:
A new animal has been introduced to the household.
A resident animal has died or no longer lives in the house.
A resident animal is re-introduced after an absence.
A young dog reaches social maturity, which is usually between 10 months and 2 years of age, and challenges the established higher-ranking dog.
A high-ranking dog ages or becomes ill and cannot maintain his higher status.
The Dominant Dog – Dealing with Dominance in Dogs Dog Behavior Blog: When Two Female Dogs In The Household Are Fighting Canine rivalry - dogs fighting with each other - Article on Pets.ca | Pets.ca How to Keep Dogs in the Same House From Fighting | eHow.com