Political correctness in the 1990s was simmering the frog
Political correctness in the 2020s is boiling the frog
It seemed harmless (and I think many, particularly younger Kiwi Farmers, forget that PC drove people up the wall back then too) because we were still at the top of the slippery slope. Indulgence and exaggeration of those values lead directly to today.
As far as race relations go, I think they peaked in America in the 1980s/1990s. During the 1960s and 1970s we were still learning to live with Blacks as equals. North and South alike. Mexican mass immigration hadn't, to my knowledge, really started yet, and nobody would care about Muslims at all until 9/11, which I think is actually part of why the neocons became so obsessed with Islam for a while, we'd been playing in their sandbox for fifty years without the average American giving much of any thought to the Middle East (see how old cartoons depict it as some cool Arabian Nights land).
By the 1980s and 1990s, I believe it had healed over for all practical purposes. Blacks were going to be resentful because there was never a world in which they wouldn't be, and why shouldn't they? American Whites will never be able to understand, on an emotional level, the fatalism, ethnocentrism and distrust Blacks have had beaten into them by hundreds of years of experience and being conned and misused. But what they could do was suck it up and be functional citizens, and the country was quickly integrating. Whites and Blacks mostly kept to themselves but there was, as many have noted, a racial detente and multiracial society was becoming common.
Mexicans were the oddballs, somewhat new and mostly ignored. Resented for taking the jerbs back then more than for spreading crime (which I suspect has a lot to do with the new Mexicans being Central Americans, who seem way more demented). They were targets of pity more than of scorn even by the conservatives, but were also a people apart. Tragic. They didn't get an easy ride back then, welfare wasn't thrown at them as readily, they were more afraid of Whites, deportation was a genuine danger.
It all started to go downhill around Rodney King, but it was still very slow. Obama blew it wide open. What we're living in now is a new Nadir of American Race Relations, the 1910s/1920s all over again but with Whites as the victim group. Last time it was shitbag Progressives trying to jail their political opponents (Eugene Debs), start wars to make the world safe for democracy, colluding with industrial elites to expand federal power and wage race and religion war to get the revolutionary proles to turn on each other (rise of the Ku Klux Klan). Today it's shitbag Progressives trying to jail their political opponents (Donald Trump), start wars to make the world safe for democracy, colluding with industrial elites to expand federal power and wage race and religion war to get the revolutionary proles to turn on each other (rise of BLM and Antifa).
It's also the 1970s all over again, too, except without the cool cars, good music, cool clothes etc., the 1970s terrorism and inflation but everything is gay and sucks now.
I don't know what exactly made it blow up. I have to think Blacks were always much angrier about things than they let on. But the media really did play a huge role in fanning the flames of race war. They knew exactly what they were doing when they'd take shut-and-dry self defense cases like Trayvon Martin and Mike Brown and try to engineer anti-White pogroms from it. Obama likewise ran on a platform that lead (braindead) normalfaggots to think he was some messiah (civil libertarian, social liberal, isolationist, puralist) and then ruled as a wannabe dictator, warmonger and sectarian.
I think it probably was, as many have said here, Occupy Wall Street and the Tea Party. The real ruling party of America, all along, was Progressivism. Had been since the days of Woodrow Wilson, that bastard, when he took the worst of the North and of the South and merged it into one ungodly abomination that has ruined the country and will likely kill it. The Progressives, an elitist and technocratic bunch, saw a very sharp threat to their power if the multicultural/civic nationalist populists of Left and Right realized that they had more in common with each other than they realized, and so they tried to stave that off (very successfully) by provoking a race war.