Starscream stayed on Earth because that's where the Autobots are, and he knew more Decepticons would come, answering the final, explosive call of the AllSpark. Come they did, and now Starscream has the beginnings of a new Decepticon army. While his troops hunt the other Autobots scattered around the planet, he attacks Optimus Prime. He hopes that surprise will be the edge he needs to destroy the AllSpark powered Autobot leader before Optimus Prime can mount a defense.