Why do dogs' back ends sometimes drift to the side when they run?

Ahoy, Matt:

I've noticed that many of our canine friends seem to walk, um, well, sort of sideways. It's like their hind legs are going faster than the front ones and are catching up. Usually it's the rear legs that are to the right of the front ones, but sometimes it's the other way around. And it doesn't seem to depend on which side of the woofer the human companion is walking. I've seen this on all sorts of breeds and sizes of our pawed pals. What gives?

-- Rob (LHR), at sea

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.