I fostered a mill bitch dobie once who would do that. She was just a weird dog though. She was fine with dogs when you were watching her... but as soon as you turned your back she would... try to intimidate them I guess...
But anyway, if you startled her she'd snap at you, but as soon as she really "saw" you, she'd stop and she'd have this "omg! I'm soooo sorry!" look on her face. She was REALLY scared up though, so I would assume that was from a life of living with lots of other dogs and fighting for food/territory related.