It's not like the masses aligning with one side or the other is a new invention, it's how politics works and always has worked. So, in a way, what's "causing it" is humanity being stupid and measures against it are attempts at managing stupidity. Nobody wants to hear that. Because everybody is too stupid to drop their ego.

That being said, there's about three articles a week that blame the unchecked spread of stupidity on the internet for the uptick in bizarre fringe ideologies. And they're probably right. One of the best explanations of how this happens I recently found in a comment thread – unfortunately I don't remember where (I assume tildes.net). In short, social media tries to find patterns in what people like and documents/encourages them. Content creators then try to make money from following those trends very quickly, which artificially accelerates and grows them. It used to take a while for some random, weird ideology take over, now it only takes one viral youtube video or tweet, even if people initially just watch it ironically.

It's no different than it always was, it just runs way too fast.