If these Wikipedia tables are to be believed, California went from 1.5 million people in 1900 to 29.8 million people in 1990. During the same time, US population went from 76m to 249m, so it's not just a matter of the US population growing as a whole.
The Dust Bowl in the 30s pushed many farm families from the Midwest out, and California was unaffected by the drought and was economically better off than much of the nation. Grapes of Wrath, by Steinbeck, covers this period.
During WWII, California was a major staging ground for the Pacific Theater, and also ramped up the existing industrial base (as did Texas, which also had a population explosion).
Once the war was over, many returning vets stuck around. California hit the relative jackpot - lots of military bases (and resulting DoD contracts), lots of agriculture, the massive expansion of Hollywood, mining, and a government interested in investing in education and research at the right time.