What started the chain of events leading to Jem's breaking his arm, according to Jem?

This is a quote from the text:

"When he was nearly thirteen, my brother Jem got his arm badly broken at the elbow [...] When enough years had gone by to enable us to look back on them, we sometimes discussed the events leading to his accident. I maintain that the Ewells started it all, but Jem, who was four years my senior, said it started long before that. He said it began the summer Dill came to us, when Dill first gave us the idea of making Boo Radley come out." (Lee 3)