The Precipitating Event

Most stories have a precipitating event: Everything is going normally, until one day . . .  something happens, setting in motion the events of the story. That "something" is the precipitating event, which I will call PE for short.

For example, Anne arrives in Green Gables, Voldemart kills Harry's parents, or a robot with a message lands on a planet. The power goes out in a park with dinosaurs. Someone is murdered, a dame walks into the detective's office needing help. Hazel meets Augustus.

Starting With

In a way, the "real" story doesn't really start until the PE. So that's a very logical place to start a book:

I'm frozen in the doorway.
He's dead.
I'm supposed to be here – Mrs. Andersen told me to get a top hat. The dead body is NOT supposed to be here.
(Dead Body Girl, Sohan)

Or the first scene is the PE.

Her father suddenly threw aside his newspaper and jumped to his feet.
and the tornado struck.
(The Wonderful Wizard of Oz, Sohan)

Less Need for Context

As noted, there is a problem of context for the first scene, and that problem can be minimized by starting with an event that is intrinsically meaningful. The PE (or anything before it) tends to meet that requirement as well as it can be met.

Why? The PE sets in motion the events of the story, they are all unexpected, they are all important context to understanding the meaning of the story. So any place after the PE has problems with context.