There is no question that the US has been monitoring the Chinese build-up of short and medium range missiles along the Chinese southern coast for some time.
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.