Well first of all, the internet has wayyy more users than ever before.
Being mad is fun when you don't really have an outlet for it.
But it's also easier than to be mad than to be polite, or to see things eye-to-eye with someone you already decided that you didn't like. Especially on social media sites where it's so easy to sensationalize any interaction you have.
It's unfortunate. When I was a kid I didn't get this sadly and wasted a lot of time messing with people and arguing.