The other reason was that this is the way that memory works. When you think about the past, you don't think about events in chronological order, but jumbled, out of order. A second root was exploring how people get obsessed with certain past events and keep returning to them. In the story, the narrator returns physically to the same event, but in real life, people get obsessed with a past event, and let that control and color the rest of their lives. I was also exploring issues of what you would do if you had perfect freedom, or alternatively, if you had absolutely no power to control the future.
Human beings take their power to control the future, even if only in a small way, for granted.
A third root is that I wanted to write a story with real physics in it. To often in science fiction, writers just make up the physics and it doesn't make sense. Since the 1920's, there have been fascinating, strange developments in quantum mechanics. I wanted to explore these fictionally. I wanted to write a story in which I actually discussed real physics.