The biggest reason is tone and the motives of the person who is disagreeing with you.
The worst arguers on the Internet don't just disagree; they bring a moral judgment along with it. "You're wrong for believing in X, and the fact that you believe in X makes you a bad person." If you have enough people who agree with this, (and this often has absolutely nothing to do with broader support for X or whether X is actually bad or not) you actively feel hated, demeaned, etc for believing in X and voicing your opinion. Even being polite will provoke an enormous amount of vitriol simply because it's very easy to lump in a polite X-believer in with preexisting ideas that all people who believe X are assholes.
A lot of people react to this with, "Well, good. There are a lot of shitty Xs that people believe, and people really should be afraid to say those things out loud."
That's all well and good for some opinions; I'm not advocating being polite to, say, neo-Nazis who are denying the Holocaust or advocating genocide. But people rapidly expand X to "whatever I disagree with," and the result is that we treat disagreement on legitimate, reasonable issues that people will naturally disagree on as equivalent to endorsing genocide and believing that the Holocaust didn't happen.
It doesn't help that the Internet naturally magnifies hatred because it's very, very easy for 5,000 people to get ahold of something and leave one really nasty comment apiece. They spend 10 seconds writing a comment, but the person who originally posted X gets 50,000 seconds of written vitriol. It's overwhelming.
As a result, there are certain issues on the Internet (and in real life, for that matter) - vital issues, important issues, that I will not touch because they are far too likely to dump a massive amount of vitriol on me. I have better shit to do. As a result, the best possible outcome for me is not to engage entirely.
And then those exact same people who dump anger and hatred on all believers in X complain that said believers in X are "avoiding them." Well no shit they are, who would volunteer for that?