What caused the sheet of paper to fly out the window in Jack Finney's short story Contents of the Dead Man's Pockets?

In the short story, "Contents of the Dead Man's Pockets" by Jack Finney, the sheet of paper flies out the window due to a sudden gust of wind. Tom Benecke, the protagonist, is reaching out to retrieve the paper when it is blown out the window, leading to a series of events that follow.