The grandfather's reaction in the morning left everyone stupefied. Why?

The short story "The Night the Ghost Got In" by James Thurber tells of a misunderstanding that erupts into full-blown chaos. The narrator thinks that he hears footsteps downstairs. He supposes at first it's a burglar but then later decides that it must be a ghost. He calls his brother, Herman, and they listen together. When his mother comes, she assumes that it's burglars and throws a shoe through the neighbor's window to arouse him to call the police. When numerous policemen arrive, they break through the front door and tear the downstairs apart looking for clues.
All this time, through all the confusion, the grandfather has been asleep on his bed in the attic. As the narrator explains, he has some sort of dementia, supposing that he is in the midst of a historic battle. When the police burst in, the grandfather fights back. He hits one and sends him sprawling, and then he grabs a gun from another and shoots him in the shoulder. The police hastily retreat from the enraged grandfather. Grandfather's reaction in the morning and the response to it sums up the fact that nobody really knows what caused all the chaos and destruction.

Over his third cup of coffee, he glared at Herman and me. "What was the idee of all them cops tarryhootin' round the house last night?" he demanded. He had us there.

The ending fits in with the comically absurd nature of the plot. Grandfather knows nothing about what has occurred other than that the police invaded his room. Everyone else is ultimately just as ignorant, because no trace of ghosts or burglars is ever found.