One of the first effects is that the only thing that gets visibility when there are 'opposed' groups (in other words groups that do not upvote one another -- this effect is even more exagerrated when we introduce downvotes) is the largest plurality. One view has 51% of the support, the other has 49%. The 49%, even though it represents practically half of all views, would be buried anytime it's mentioned. You'd expect there to be about a 49% chance of any comment you read of being a 49% view. Instead you'll read each and every single 51% comment until you get down to the bottom sorted 49% views. Consequently we see groups begin to splinter off into their own little spaces where they can discuss things separate from the 51%. And just like that you have fermenting radicalism.
An analogy I quite like is to imagine we're trying to solve a math problem. The correct answer to this problem is 0. However, one side believes the answer is absolutely at least 50. And the other side believes the answer must be no more than -50. When these two groups remain within contact, they counter balance each other. But now let's isolate these groups. In the past when the -50 side chose to go -60 there would be some push back against that and it would help create a more of a central equilibrium. But in isolation without opposition, -60 sounds awesome. Why not -70, or even lower? And on the other side an equal but opposite push for 60, 70 and more is simultaneously happening.
The more segregated we are, the more out of touch with reality we become.