Fowler jumped at the sudden knocking at the door. Ausable just smiled. "That will be the police." he said, "I thought that such an important paper as the one we are waiting for should have a little extra protection. I told them to check on me to make sure everything was all right."
Why did Fowler jump at the sudden knocking at the door?

Explanation

Fowler had already seen Max intruding with an automatic pistol in his hand. When he heard a sudden knocking at the door, he jumped up in surprise.