I don't think anyone knows exactly what happened, they say someone kicked his leg. Funny, the top youtube comment reads, Klitschko - "if someone uses physical force, there will be a reaction!!" Klitschko gets hit. Klitschko - "if someone uses physical force AGAIN, there will be a reaction!!!"
Let's face it, Vitali just couldn't afford to react physically, almost no matter what the provocation. Any footage comes out of him actually hitting someone and his political career's probably knackered. Wouldn't surprise me if it was deliberately done for that purpose, or if it happens again, for that matter.
http://www.youtube.com/watch?v=0GL93Ky035Y I had never seen the entire scene, big Klit is not joking around.