Why does my dog run sideways?

Very odd phenomenon, watching a dog that looks like he needs a back-end alignment. And very hard to find a vet who will admit to knowing what we're taking about. But in our own irritating way, we kept at it until one of them finally gave up the info. Did you know that your dog has dominant and nondominant sides, ergo, dominant and nondominant legs? That if he could use a pencil or a spoon or a pistol or toilet paper, he'd favor one paw over the other? How'bout that. Anyway, the odd butt drift comes because the stronger, dominant leg pushes off harder when the dog runs. To use this power efficiently, the strong leg gradually moves into the midline position, between the front legs, as the dog builds up a head of steam. You see it most when Rover's moving at a fast trot or a loping gait. When he's at a dead run heading for the mailman, it's not so obvious.