When I had a situation like this - when one lashed out at another without any warning, it ended up that Sammie was VERY sick with cancer and we had no idea. She was apparently in pain and all Nicki had to do was look at her and Sammie would be lunging. I feel AWFUL for not recognizing Sammie's illness - not that its an excuse, but hubby's mom was dying of cancer at the same time, and we were a little preoccupied with MIL (who died two days before Sammie did )

Have both dogs been ot the vet for a thorough checkup? One might be sick and you might not know it. From what I've heard since my experience with Sammie, this scenario is rather common when one dog is sick withotu the parents knowing it.