It's quite common on any race cars that when the rear tires or front tires becomes to stressed it starts to shake another example, http://www.youtube.com/watch?v=_wpmuzSBuV0 a trained eye can see some sort of 'wobble' or tire looking unstable, and it doesn't take much more than a small little dent in the road for a tire to get off balance, and it doesn't mean much in handling, most people don't actually know it's going on.
Here's an onboard view from my camaro, and you can see quite clearly how the tire behaves : http://www.youtube.com/watch?v=h8HJmfYxgH0