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.