The very beginning lines of the book set up a conflict between Jem and Scout: what caused the event that led to him breaking his arm? Scout argues it is the Ewells that started it, and Jem argues that it goes all the way back to when Dill came to visit and they all wanted Boo Radley to come out. The two distinct story lines in the book that come together in the end (obsession with Boo and Tom's trial) make it very difficult for the reader to discern which event actually started it all. Perhaps that is Harper Lee's intent; to show that a chain of events cannot be separated out to determine fault or guilt - it is a culmination. Both events came into play in how Jem's arm got broken; Bob Ewell never would have been after the kids had it not been for Tom Robinson's trial and Boo Radley might have never come out to save the kids had it not been for their obsessive interest in him during earlier summers.