China has a history of conducting itself unprofessionally and recklessly on the high seas and in international airspace (look at the EP-3 collision as well as Chinese harassment of US oceanographic survey ships). It's been using its relatively new found status as a regional power to act like a bully in the Pacific. A reaction like this to our presence in the area (which we obviously wanted them to know about if we sent a CG to the area) is not surprising.