TS explains that, when the accident happened, he screamed so loudly that his mother came running. His father came, too, but he already knew what happened when he got upstairs.
As Susannah fled the scene, she told the story to the cook, who told it to Jonathan, who told it to Obadiah, who told it to Mr. Shandy in one crazy game of Telephone. Hearing the story, Mr. Shandy says, "I thought as much."
Yeah, you might think Mr. Shandy had already written the best chapter in the Tristra-paedia, about forgetful chamber-maids and sash windows, but in fact he hasn't: if he had, he would have nailed the window to prevent the accident.
Also—as it happens, Tristram wrote the chapter himself.