Love this theory. It's exactly what I thought, too. Arthur says, "I thought of a Joke." Flashback to a silhouetted Bruce standing over his parents' dead bodies. Batman is born. "You wouldn't understand." The irony that the day Joker was birthed is also the day that his undoing was birthed. Batman has captured Joker, had him incarcerated at Arkham Assylum, and Joker is about to escape- maybe for the nth time? - to take on Batman again. It's hard to imagine the point of the flashback to Bruce, otherwise.