China has a long standing conflict with Taiwan. Many analysts believe a future military conflict is brewing. China has made several threatening gestures and statements directed at Taiwan. So, this reason alone is sufficient to understand why China would risk an international incident in harassing the P3 as it monitors Chinese military activities. Consider another reason for a moment.
China is a major provider to Iraq for military equipment and support.
In the recent air raids into Baghdad, the allies were attempting to destroy the new Iraqi missile command and control system. This was a fiber-optic system designed and built by the Chinese. Many of the Iraq armored vehicles are Chinese made. Iraq is on its own campaign of conflict, teaming with Syria and the Palestinians, for a future war into Israel. However, Iraq knows that the US can do major damage to Iraq in its attack upon Israel.