Scout says that Atticus’s return was the second reason she wanted to quit playing. What was the first reason?

From the text:

Atticus’s arrival was the second reason I wanted to quit the game. The first reason happened the day I rolled into the Radley front yard. Through all the head
shaking, quelling of nausea and Jem-yelling, I had heard another sound, so low I could not have heard it from the sidewalk. Someone inside the house was laughing.