Well, most of the power for the pounce comes from the hind legs. The butt wiggle is actually a shifting of the back feet in place, if you observe closely when it happens, as I have with my kitties. Very similar to us testing a loose-looking rock or sheet of ice to see if it will shift under our weight or not support us.

So I believe they do it to make sure that when they release all that energy to catch their supper, their footing won't slip under them, throwing off their pounce and missing that hard-to-catch meal.