It's mostly the internet. It existed to some extent before, the media always pumped out fake news to protect narratives, but instead of being more careful now that people can fact check them, they've actually turned it up to 11 and become more steadfast in their dishonesty.
Couple that with people now not only interacting with each other who probably wouldn't have before, but doing it semi anonymously where you can be as rude as you want and say shit you would never say to someone's face.
You can see the difference in Obama's 2008 campaign vs his 2012 campaign that coincides with all of this. 2008 was mostly about his left wing policies, and 2012 it turned into almost entirely identity politics after his first term brought the worst economic recovery in 100 years and the middle class was becoming the minority. It turned into crying about Fox News, calling everybody who doesn't support him a racist, and "Trayvon Martin could have been my son". Then of course a year into his 2nd term after getting substantially fewer votes than 2008, the middle class finally did become a minority, BLM sprouted up with the "hands up don't shoot" lie, and anybody who didn't like it was called a white supremacist.
