Why does Framton Nuttel leave in such a hurry and how does Vera explain it?

Mr. Nuttle leaves in a hurry because he is frightened. Vera has told him a story about why Mrs. Sappleton leaves the window open, saying that Mrs. Sappleton believes her missing husband and brother will return thought it, just as they left through it, seemingly long ago. Although the story is not true (the men only left that morning), Mr. Nuttle, a very nervous guy, believes it. Therefore when the men do return through the window, Mr. Nuttle suffers a "nervous breakdown" and must flee.