We don't know much at this point.Was it stated that this game was to take place after the events in TFU? If not it's possible it could take place during the first one.
Maybe the trailer is showing an event that took place during Starkiller's life that ended up setting the stage for whatever occurs in the second game.
He may not even be the main character.Maybe what happens next introduces us to a new character and they were only showing the part with starkiller to gain our interest.
Your guess is as good as mine.