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.