I would say the biggest factor is the emboldening of the people who were already toxic, because this has been progressing for decades now in the US, if not the rest of the world.

Once the racists could hide their hatred in plain view by simply saying “it’s just a joke”, it became that much easier for them to say the quiet part out loud. And once they got bold enough with that, all bets were off. The divisiveness of media (both traditional and social) is profit-motive driven and probably comes as a direct result of the former developments. Clicks bring in the ad revenue, and nobody clicks faster or more often than an angry crowd.

There’s plenty of other factors too, I’m sure, like lead poisoning from all the lead being spewed out by cars with leaded gasoline back in the day (one of the side effects of lead poisoning is increased aggression), and the removal of third places that had once allowed people from all walks of life and social classes have a common area where they’d meet and interact. But letting the racists get away with it is definitely a big one.