Well that's the weird thing ... she wrote it before To Kill a Mockingbird. It was apparently and early draft, and she rewrote it to focus on Scout as a child. She may have had no intention at all to release it, but her mind was kinda going at the end of her life.

Her sister Alice kept people from taking advantage of her, but after she passed, she was probably taken advantage of by her lawyer and her publisher to push the book out as a cash grab.