Why was Washington's attack in new jersey a surprise?

It was a suprise because the Contidental army was pushed back. Since the Britsih and the German under their control were camped in Trenton, the army could take them out which could help their efforst in the war. So, the army took boats across the river at night to not be seen and attack at early morning. It would be a supris then beause the Britsih were not expecting them to be attacked at 8 Am in the morning from the Contidenal army.