The event in this chapter that was inevitable was the December Riot. The narrator says that you could really see it coming -- it was obvious something like that was going to happen.
The reason the riot was obvious is because it was a whole year after the Pearl Harbor attacks and the pressure on the internees had been growing and growing. The men had been getting more and more upset. They feel angry and helpless because of what has happened to them. They have been holding lots of meetings talking about their grievances, and by December, the anger boils over.
In chapter 9 of the book Farewell to Manzanar the Japanese Americans have been encarcerated in camps in America due to the war with Japan. All rights have been taken away from the people because of their heritage and race. They are placed in encampments that are dry dusty and boring. One day is as dull as the next.
The narrator is telling the story of when she was a little girl, named Jeanne Wakatsuki, and placed in the camp. She discusses how disheartening it was to her papa for them to be charged with disloyalty. The bitterness and rage builds up in the people until it leads to the December Riot. The riot is what the narrator says was inevitable.