Dogs just love to wag their tails. We typically associate a dog wagging its tail with happiness, but the truth is that dogs use the extremity to convey a variety of strong emotions. In fact, a dog wagging its tail can also happen just before it bites something. Tail wagging is actually only half the story, with a dog's posture being the other half. Differences in stance, the direction the ears are pinned, and facial expression can mean everything from happiness to annoyance.
The basset hounds in this video are strangely wagging their tails while lying next to each other. Their owner describes the two as a couple of clowns, and it isn't exactly hard to see why.