Why did Zidane headbutt Materazzi

On July 9, 2006 in Germany, Berlin, the World Cup final between France and Italy was being held at Olympiastadion stadium. 7' Zidane scored a goal from a penalty kick, 19' Materazzi's goal assisted by Pirlo. First half ended 1-1 draw and no goals in second half, went to extra time and then the 110th minute… Zidane headbutt Materazzi leading to red card for the French team's star, the rest is known. I don't know how to describe this incident from the spectators' point of view, bizzare? shocking? What the hell is going on here? Spectators were watching the two teams heading to extra time to fight for the beautiful cup, a contact between the scorers, then surprisingly Zidane headbutted Materazzi. We can say that this moment stole the lights from the tournament itself and don't forget it was Zidane's last match so the fact that red card was the last moment of his career added more drama to the story. What did Materazzi say to Zidane? Many rumors claimed