Sarah knew that Biff just needed some time to reflect on what happened (as did Sarah himself). He decided to go somewhere for a while and then go back to Biff's house, then maybe try to figure out what happened. He figured that going home was a bad idea, since his parents wouldn't recognize him. Meeting up with his friends probably wouldn't be a good idea either, since they wouldn't know who he was either. And it would be difficult to explain to them, without them thinking he was nuts. No, he'd have to go somewhere neutral, somewhere where he could think but where no one knew him.
And that place, he decided, would be ...
