There's a few folks on digg that said it was cause Materazzi gave him a nipple pinch and was insulting his mother, etc.  Don't know how credible it is but you can see some mischief by Materazzi in the very beginning of this clip: http://www.youtube.com/watch?v=7C58zxHgo8Y . I don't think anyone honestly feels it was unprovoked but it may have been more than just words from the other guy.