For each book, there's a back story of where the idea came from. Sometimes it's derived from a current event or topic of discussion. Often it begins with a character. And often, I have NO idea what sparked the idea. It's just there.
We grown-up people think that we appreciate music, but if we realized the sense that an infant has brought with it of appreciating sound and rhythm, we would never boast of knowing music. The infant is music itself.