Why do you think Lee starts To Kill a Mockingbird by mentioning Jem's accident in the 1930s and then flashing back to events taking place before the Civil War?

The book opens by mentioning how at age twelve, Jem broke his arm. The narrator notes that the remainder of the book will explain how this injury occurred, and the novel concludes with this event. From the outset, through historical analysis, the novel tries to conclude how "this particular situation" arose.