I have had better luck with this action in the morning when their energy is high, so it might have less to do with the drink, and more to do with a high level of energy.
That's what I think, too. If they've got a high energy level the peeps will look for ways to work it off by various forms of exercise. Kung Fu is just one way they can choose.
I've also noticed that they seem to do Kung Fu in the morning.