Given that the US doest not have an official language, this rings a weird bell. Had the been England or any other country with an official language, it would have been something. In particular with the history of California and the proximity to Mexico, the opposite (that more than 50% speaks english) should be the story.
California was almost empty when it was annexed into the US. Disease etc. Immigration from the rest of the English-speaking US completely dwarfed what was left. Almost 100% of Spanish-speaking Californians have come in the past 50-70 years. Basically none have historical roots to California.
What is known as the American state of California was part of the state of Alta California in Mexico, which was annexed by the US during the Mexican American war.
https://en.m.wikipedia.org/wiki/Alta_California
Years later, the Mexicans as well as their descendants, who were Americans by Jus soli (birthright citizenship) were ethnically cleansed during the Mexican Repatriation, where they were kidnapped, put into buses and trucks and sent to Mexico.
https://en.m.wikipedia.org/wiki/Mexican_Repatriation
Many Mexicans are mestizos (mixed with Native Americans), so they have a deeper connection to the land dating to tens of thousands of years and this is reflected in the population genetic haplogroups.