How does the fight begin between the demonstrators

As the story opens, Sam is in a demonstration led by his father, Roland Childs. Sam's mother, Marjorie, is near Roland as are his two body guards, Ty and Jerry. The family has recently been threatened because of Roland's civil rights activities which prompted the body guards. The demonstration is taking place in Chicago and there are many people gathered. Sam is standing near his brother, Steve Tyrone, who Sam calls "Stick." When Sam and Stick are momentarily separated, Sam panics. He knows that, at thirteen years old, it's a ridiculous reaction and he doesn't tell Stick about it. Stick asks if Sam would like to leave and he's tempted but knows their parents always know if they try to sneak out, even in the large crowd. It's cold and Sam eventually gives in to the discomfort and agrees to leave. The boys are near the edge of the crowd when a fight erupts. An older woman is being beaten to the ground and Stick jumps into the fight. A white man uses a broken beer bottle and cuts Stick's temple. The boys begin to run but Stick is soon exhausted from blood loss and Sam knows he has to get Stick to the emergency room.