The internet has brought communication to everyone that travels at incredible speed. This has the effect of causing emotive responses, rather than reasoned. Requires no effort at all to respond, and encourages the toxicity you describe. What gains most traction are those posts that are the most emotive. It used to be "if it bleeds, it leads", but I suspect these days it's more akin to "emotion deserves promotion".
Speed carries a number of other implications. For example, it allows a cheap, steady stream of constant information that can be completely irrelevant at best to you, plus it's information about which you often can do nothing about, except hit the reply button. This can only make things worse, because people feel powerless except in this superficial way.
The other effect is the quantity of information, and it's impossible now for a user of the internet to wade through it, or even validate it without incredible effort. Our current techniques for filtering that information is essentially populist individualism: we rely on what other people have liked, linked to, or shared, but at the same time, read what is presented to us based on our previous interactions.
I guess what I'm trying to get at here is that I perhaps pessimistically feel that these things are fundamentally baked into the medium, in much the same way that Neil Postman felt entertainment was intrinsically baked into television. Hopefully I'm wrong, and there are solutions to these things. Not sure what they are myself.