Confirm thy soul in self-control
To those who speculated that something relating to him being a Muslim was said, it may be but Zidane himself has said that he is a "non-practicing Muslim." What does that mean? It could mean all sorts of things, but one thing that is implied in such a statement is that being a Muslim is not as key and core to his self as some might associate with "devout" Muslims, some of whom (some, not all) might find derogatory and insulting comments about Islam, their prophet, or Allah as good enough reason for retaliatory actions, like a headbutt. I had thought of this as a reason but was not too sure how dear his faith is to him. If dear (however much he is a "non-practicing Muslim), I could see why he might react as he did. If not, then why? Perhaps it was rather something else--though to some a related reason--like a statement against his Algerian heritage. Some in the French press are saying Materazzi made a comment about Algeria and/or terrorists. Time will tell.
For more on his background, you can read "ZZ Top," a detailed article about him which begins:
Zinedine Zidane's journey from the rough back streets of Marseille to Madrid has been marked by racism, political controversy and superlative football. The world's best player tells Andrew Hussey of his pride in his Algerian heritage, his rage to be the best - and reveals why his talent can still be engulfed by flashes of violence.
For now, Forza Azzurri!