It might be the body language that the other dog is giving off that sets him off or a combination of the signals both dogs are giving off. I read a book about animal communication that specifically mentioned that Cavaliers are so far removed from their ancestors (wolves) that they have lost almost all of the traditional dominance and submissive signals that they show towards each other and tend to get into trouble because they give off inappropriate signals and/or don't pick up on other dogs signals. If I recall correctly this study concluded that Cavaliers have only 2 of the 10 dominance/submissive signals and they are both dominance. The study also suggested that dogs that look more closely like wolves are most likely to have the least problems dealing socially with other dogs.
I'm not sure I totally believe it but it did make for some interesting reading and I try to be aware if my guys are staring (we train with border collies so I know that is a precursor of trouble) and just generally be aware of everybody's body language.
Mindy Tri - Feb/97
Max - Ruby - Sep/08
Rylie - B&T - June/09