A lot has been made of the former president Donald Trump's narrow upset wins in three Rust Belt states which won him the presidency. This was part of a long term trend that continued into 2020, as I describe below.
Trend 2004-2020 president (trend is shift adjusted for average):
You can see all but one Southwestern state move towards Democrats relative to the nation. (Nevada narrowly did not.) You can also see parts of the Southeast and Northeast trending blue too. (I don't think including DC is fair because of how heavily Democratic it is, but it still barely trended blue!)
This trend continued albeit at a slower rate in 2020 with Ohio and Wisconsin trending towards Republicans vs 2016, and Texas, Arizona, and Georgia trending towards Democrats.
Because we adjusted for national popular vote shift, the number of people living in a state trending towards Democrats is about the same as the number living in a state trending towards Republicans.
What is causing this long term trend that has been present since at least 2016? (I know there are other areas trending this way, I just couldn't fit them in the question.)