I don't wake up thinking "today I want to be angry", but if I go outside and see somebody kicking a cat, I feel that anger is the correct response.

The problem is that social media is a cat-kicking machine that drags people into a vicious circle of anger-inducing stimuli. If people think that every day people are kicking cats on the Internet, they feel that they need to do something to stop the cat-kicking; given their agency, that "something" is usually angry responses and attacks, which feeds the machine.

Again, they do not do that because they want to be angry; most people would rather be happy than angry. They do it because they feel that cats are being kicked, and anger is the required moral response.