First theory
Step 1: 'Our site is full of people that disagree with each other. Let's call one side <insert label here> and get the mods to ban them.
Step 2: The targeted people are either a) pushed off the site or b) pushes back, doesn't get banned, and the attacking side leaves.
Step 3: Whoever left the site creates or goes to a site that caters to them.
Result in either case: Two sites full of people who only agree with each other.
Second theory
Step 1: Hey if we call our opponents <insert label x, y, z>, harass them, then take any push back as confirmation of our views, we never have to engage with them. We can just silence them and then we win forever!
Step 2: Oh shit, people suddenly hate us a lot more than I remember. Oh shit. There's only one natural conclusion to draw from this. WE WERE RIGHT ALL ALONG. THERE WERE JUST A LOT MORE <insert label x, y, z> than I remember.
Step 3: Because people grow to loathe us over time, we loose influence. The only solution is to escalate our suppression of dissent.
Result: Negative feedback loop of -> suppressing dissent allowing dumb ideas to grow, causing more dissent, causing more suppression, causing even dumber ideas, repeat ad nauseum
Third theory
There are a intrinsic traits of online communication (mostly text based, large scale, not in real time, takes time and effort to do) that inevitable leads to people being more hostile and arguing with each other. Over time this naturally leads to people only using services and places where everyone have a high degree of consensus and they only encounter information that validates their beliefs.