There are two possible answers to this question. The first is his sister Helen's choice to run away. At 15, she had become an activist, preaching to her siblings about the "revolution." One night, she fights with Rosetta, "the resident queen of the house." The resulting brawl sends Helen fleeing from the home, only to return 5 years later, with "a nursing degree & a baby girl." Her refusal to come home drives Ruth to tears and near hysterics, & disrupts the hierarchy among the siblings.
The second possible event is the death of James' stepfather, which occurs when James is poised to become the oldest child at home, & hence "finally the king of the house." But his mother is destroyed by his stepfather's death, & she loses control, both psychologically & within the household. James' grades plummet, he experiments with drugs, & he begins committing petty crimes. Essentially, the structure in the lives of all breaks down.