When I play tug with my 9 week old GSD pup he humps after winning the towel. Is this a worry?

I have a 9 week old male pup from strong working lines. had him a week. Really super, level headed, drives that us working folk are looking for. One snag though. When playing with a towel, rag, old shirt, he has started humping it? after winning it. Is this a worry, what action should I take. So far I distract him and continue the game. I'm following Micheal Ellis systems on training. Would appreciate some sort of guide.Thanks