The correct answer to this open question is the following.
The focus of the civil rights movement shifted away from nonviolent to more militant forms of protest in that for some protesters, it was a way to defend from the police aggression. After maintaining non-violence civil rights demonstrations, many people suffered from the aggression of the police or other people that did not support civil rights ideas. Tired of suffering from those aggressions, many civil rights activists supported the idea of reacting violently or started to act violently as a way to justify self-defense and put a stop to the aggressors.