Why were the Japanese Americans moved off Terminal Island?

In December 1941, the Japanese attacked Pearl Harbor, bringing the United States into the Second World War. Among other things, the Japanese attack created a great deal of animosity and prejudice towards Japanese-Americans. Even though the vast majority were hard-working, law-abiding, and patriotic, (like Jeanne's father in the story) the Japanese-American community came under intense suspicion. Many people falsely believed that they were potentially disloyal, traitors who would somehow assist the Japanese war effort against the United States.
In a climate of growing suspicion, paranoia, and prejudice, the Roosevelt Administration initiated a policy of forced internment of Japanese and Japanese-American citizens. Whole families were uprooted and moved to settlements where their movements could be more effectively monitored by the authorities. Some people were moved to ghettos; others to what can only be described as concentration camps. In Farewell to Manzanar Jeanne's family, along with other Japanese and Japanese-American citizens, is moved from Terminal Island to the immigrant ghetto of Boyle Heights in Los Angeles.
The rationale of this program of state-sponsored forced relocation was that it was strategically necessary to protect the Long Beach naval station. In reality, however, there was no imminent threat to national security from people like Jeanne's family. They were largely the victims of racist mass hysteria.