Something finally occured to me, Atlas has been displaying periodic leash-aggression and I didn't really pinpoint until now- it is only when my girlfriend is present. In fact, even when he was a puppy he would bark at people when the three of us would walk together but not when it's just with me.
I don't think it's that he's posessive of her, because for all intents and purposes he is "my" dog (rather than hers). He's my shadow at home, if he was going to be posessive of anyone it would be me. But he only shows aggressive tendencies when she's out with us.
Thoughts?