The mass migration has been going since before 1986, when Republicans gave amnesty to illegal immigrants, on the promise of getting better border control [1]. They didn't get it, nor was legal immigration reduced in any way, nor did the Republican party itself fight too hard for either. No surprise voters felt betrayed. California passed a referendum to stop literally funding illegal immigration, only for it to be judicially overturned [2].

Again and again and again, any kind of limit on immigration, no matter how popular, was rejected. Is it any surprise it came to this?

[1] https://en.wikipedia.org/wiki/Immigration_Reform_and_Control...

[2] https://en.wikipedia.org/wiki/1994_California_Proposition_18...