It's a natural consequence of a highly-connected communication graph. People with extreme political views talk about politics a lot, so casual political conversations are naturally going to disproportionately represent the extremes. Similarly, when people more-or-less agree, they tend to discuss politics less (because there's only so much preaching the choir that people can stand). Most meatspace communication graphs happen between people who tend to have similar world views, but online, you've got neonazis and anarchists replying to the same reddit threads. The combination of the above two factors means that by far the most political views you'll end up seeing are 1) your own, followed by 2) extreme versions of your opposition's views.

Just look at any form of social media. Many, if not most, people seem to seek out and enjoy drama. A straight forward representation of some event is usually going to get orders of magnitude less "engagement" than a excessively hyperbolized and sensationalized representation of it, even if the latter may often play a bit fast and loose with the facts to further magnify the dramatic effect of it all.

And then with every sort of algorithm and feedback mechanism (e.g. upvotes, likes, etc) based on maximizing "engagement" you then get this stuff spreading everywhere and even further drowning out any sort of rational discussion. So people who regularly follow it are going to be living in some sort of alt-reality all the while convinced that they are the most informed about the latest happenings in the world.