In the early days of my haunt, I can recall creating some basic information regarding the logic behind my display's layout - a story to justify why props ended up where they ended up. I didn't share it with anyone. It was just something I kept in the back of my mind to help me with the flow of things (and to have a comeback if my father asked an annoying/condescending question like "Why ain't that skull coming back to life like them dead people are?").
Back in the late 90s, when I first added a Witch to my display, things really clicked when she became the logic behind the rising corpses. She was responsible. She was causing it (and loving it). Still didn't share it with anyone though. It was just something in my mind like a private joke. But it made things feel natural and logical.
My scarecrows have all benefited from a backstory. Back in 2005, I had the fun of inventing a Bigfoot-like creature called Roots and received emails asking me if it was an actual legend. My Bog Man's faux history is something that I still enjoy reading about, as if it were actually a real tale.
Having a backstory evolved over the years into something that I personally MUST have in order to feel comfortable with my yard haunt. The Three Sisters raising zombies from an old cemetery. A Wrath Poltergeist forming as an angry byproduct of reanimated corpses - filled with hate and rage and violence. The return of the Sisters with a new plan: fusing souls of the departed with their rotting corpses to create the Ghost Dead - forcing ghostly zombies to do their bidding (more powerful than regular zombies, by the way - and a gunshot to the brain won't help in the least). A field of mysterious scarecrows in a lonely and dark hollow. And then a repulsive legend of something called a Corn Witch.
For the most part, people visiting the haunt will never know your backstory. And that's just fine. It's really for you anyway.