What is the initiating event in "The Open Window"?

The initiating event in "The Open Window" is the arrival of Framton Nuttel at the home of the Sappletons. He is a complete stranger who comes with a letter of introduction from his sister, a woman whom Mrs. Sappleton may not even remember. Obviously, Mrs. Sappleton is caught off guard. She has to enlist her niece Vera to stand in for her while she gets ready to receive their visitor. The entire story is told through Framton's point of view. He is not a very attractive guest because he is so wrapped up in his nervous disorder. Vera probably doesn't like being dragooned into serving as a substitute hostess at a moment's notice. She may have been interrupted in her reading. The mischievous fifteen-year-old girl decides to play a practical joke on the visitor, and in doing so Vera reveals a lot about the dreary routine of the Sappleton household. The men never think about or talk about anything but shooting birds. Her aunt seems housebound and has probably rarely heard much talk about anything but birds; consequently that seems all she ever talks about herself.
Mrs. Sappleton is devoted to her husband and her two young brothers. Vera knows her aunt will sit there staring at the open window while she talks about birds to poor, ill-at-ease Framton. Her aunt will be looking forward to the arrival of the three hunters for tea. But Vera has spun a story about how the three hunters died three years ago when they were sucked into a bog. Everything her aunt and the three hunters say and do ties in nicely with the girl's ghost story. Framton must believe that Mrs. Sappleton is mentally deranged, as Vera has told him. 

"Poor aunt always thinks that they will come back someday, they and the little brown spaniel that was lost with them, and walk in at that window just as they used to do. That is why the window is kept open every evening till it is quite dusk. Poor dear aunt, she has often told me how they went out, her husband with his white waterproof coat over his arm, and Ronnie, her youngest brother, singing 'Bertie, why do you bound?' as he always did to tease her, because she said it got on her nerves. Do you know, sometimes on still, quiet evenings like this, I almost get a creepy feeling that they will all walk in through that window - "

When Mrs. Sappleton suddenly cries, "Here they are at last!" and Vera is looking at the open window with a faked expression of "dazed horror" on her hitherto "sell-possessed" young face, Framton reacts with terror. The three returning hunters, who have only been gone for the day, look as if they must be Mr. Sappleton and his wife's young brothers returning from the dead.

In the deepening twilight three figures were walking across the lawn towards the window, they all carried guns under their arms...

And Bertie makes their identities certain when he breaks into a song that only he could know.

Noiselessly they neared the house, and then a hoarse young voice chanted out of the dusk: "I said, Bertie, why do you bound?"
Framton grabbed wildly at his stick and hat; the hall door, the gravel drive, and the front gate were dimly noted stages in his headlong retreat. A cyclist coming along the road had to run into the hedge to avoid imminent collision.