Why does Tom react as he does when he sees his yellow paper go out the window at the end of "Contents of the Dead Man's Pocket"?
As the highly valued yellow sheet blows out the window for the same reason that it has done so the first time in which Tom has risked life and limb to retrieve it, a burst of laughter issues from Benecke at the existential absurbity of the moment. For, having had a revelation during the crisis of standing on a ledge eleven floors in the air with the window to his apartment slammed shut, Tom has reassessed his values and decided that his relationship with his wife Clare is of much greater importance in his life than succeeding in business. Fortunately, his desperate attempt to enter his apartment is successful.
After his re-entry into his home, Tom
...did not lie down on the floor or run through the apartment, as he had promised himself; even in the first few moments it seemed to him natural and normal that he should be where he was.
His common sense has returned to Tom. So, when he sees the yellow sheet again fly out the window, Tom understands the insignificance of his having risked his life for mere facts and figures and the absurdity of basing his existence upon some kind of monetary success.
Tom reacts with laughter because he realizes the important life lesson he has just learned. At first he believed that paper to be very important. After risking his life to retrieve it, he realizes that he has wasted his personal life and wasted time with Clare by worrying about getting ahead at work. Now, he is willing to let the paper go and leaves to find his much cherished wife.