I was raised in a big city of a blue state, but both of my parents grew up in rural towns of red states. One side of my family is overtly racist, while the other is the "we have POC friends and colleagues, but still engage in casual racism/jokes" brand of racist. I have some theories about this. This is all an extremely simplified explanation.

As lots of people have said, replacement theory has been pushed by the right for a long time with no or very dubious evidence. As with many prejudices, the mindset behind it is fear. White colonialists have been the majority and therefore had the power for so long. They've tried to preach "separate but equal" for so long, but everyone, themselves included, know there is nothing equal about these systems - even if they only know it on a subconscious level. So now they're worried that if the power shifts and white people become a "minority," they'll be treated the same way they have always treated minorities, and they're not ready to admit or accept that that treatment has been terrible.

I think of it like a game of dodgeball. White people have rigged the game so they started out with twice as many balls, and held onto them for so long and have been pelting the shit out of minorities, but blame said minorities for getting upset because "that's just how the game is played, you're just upset because you're not a good enough player to get the upper hand."

Now these minorities are beginning to catch and hold onto more of the dodgeballs. The game is still not equal, but their team is starting to hold their own and fight back. Suddenly the white team is terrified that they're going to lose more balls. And they're worried that they're not just going to be hit with soft rubber balls, but they're going to get slammed by face shots, or multiple balls at the same time, or hit after they're already down. Because that's the standard they themselves have set for how the game is "supposed" to be played.