"We didn't jump the border; the border jumped us" refers to the change in ethnic status of some:

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This phrase is often used to describe the experience of Mexican Americans, particularly in reference to the territorial changes following the Treaty of Guadalupe Hidalgo in 1848, when Mexico ceded a large portion of its land to the United States. This resulted in many Mexicans suddenly becoming U.S. citizens without moving, hence the saying "We didn't jump the border; the border jumped us."

"We Didn't Jump the Border; the Border Jumped Us" Refers