It appears he was partially influenced by his witnessing of US bombings in Sadr city.
Friends said Zaidi covered the U.S. bombing of Baghdad's Sadr City area earlier this year and had been "emotionally influenced" by the destruction he'd seen.
Zaidi's channel is Baghdadiya Television, which supports the insurgency. It appears it's an Iraqi channel, but is based in Cairo, Egypt, presumably because Iraq itself is too dangerous.
From the Times
Hitting someone with a shoe is a particularly strong rebuke in Iraqi culture. Although the president was uninjured, the incident overshadowed media coverage of the trip in the Arab world. And it has transformed Muntader al-Zaidi into a symbolic figure in the debate about the American military’s presence in Iraq.
Maythem al-Zaidi said his brother had not planned to throw his shoes prior to Sunday. “He was provoked when Mr. Bush said [during the news conference] this is his farewell gift to the Iraqi people,” he said. A colleague of Muntader al-Zaidi’s at al-Baghdadiya satellite channel, however, said the correspondent had been “planning for this from a long time. He told me that his dream is to hit Bush with shoes,” said the man, who would not give his name.
Muntader al-Zaidi appears to have a long-standing dislike of the United States presence in Iraq. He used to finish his reports by saying he was in “the occupied Baghdad.” His brother said that he hates the occupation so strongly that he canceled his wedding, saying: “I will marry when the occupation is over.”
Maythem al-Zaidi said that his brother is politically independent, but several people who know him mentioned that he was a Baathist who turned into a Sadrist after the war.