Why did folks flee to the North?
To get away from the Aztecs.
The prominent Culture was based on Human sacrifice.
The Spaniards came over, looked around, and put an end to the “prominent culture”
Or, having no actual border to worry about , the civilizations in Mexico extended northward into California and Arizona
There have been migrations of people north and south of what is now the US-Mexican border for thousands of years. Long, long before there was an “Aztec” political entity at Tenochtitlan, which was quite recent, only 150-200 years prior to the Spanish conquest.
And there was a Nahuatl (the language of the Aztecs)-speaking cultural continuum all over central Mexico for maybe a thousand years (probably more) before Tenochtitlan. The Toltecs spoke Nahuatl.
And there was human sacrifice and cannibalism as well, likely for thousands of years, both north and south of the current border. Ref. “Man Corn”, C.Turner - scary stuff.