California is not an innocent place. Think of the terrible cruelties against the Chinese. Early in the century, striking workers were shot by the police and they died on Market Street in neat pools of blood. And don't forget that L.A. had seen murder in the 1940s, when Mexicans fought the police. There are many grievances in a place as big as this place, and sometimes those grievances are not contained.
People go crazy.
A rock is flung through a window, the mob senses its power when the initial offense goes unchallenged. The mob feels its muscle swell. The police are surprised at first, and then in awe of the swelling. And then the police turn angry. These are not matters for an old man to remember.