Having been in front of an Internet hate mob or two, the mechanism is pretty simple but devilishly cumulative.
YOU are a private citizen that feels disempowered. Perhaps you graduated college and can't find work. Perhaps your parents grounded you for being late when it totally wasn't your fault. Perhaps your boss didn't recognize all the work you put into that report. Hell, perhaps you were four over par yesterday.
I am a private citizen that, somehow, is a public citizen in your mind. I could be a CEO you read about on Gawker, I could be a moderator on Reddit, I could be someone 4chan mistakenly believes tortures animals. The important part is that we're equals, but because we don't intrinsically understand The Internet you think we're not.
Your response is to pillory me. You do this the same way you shout at traffic through your windshield - from your perspective, there is no difference between the front seat of your Honda and the office chair in your cubicle. After all, the internet is an echo chamber of incoherent rage; what's one more howl in the woods?
This is how wolf packs form - one dog barks and the whole neighborhood choruses. So I don't know you, you don't know me, and inside 6 hours I've got 900 death threats for being rude in the modmail.
It's funny. My internet profile is pretty well sanitized. Call it basic OPSEC: I moderate a default subreddit and have said controversial things on large forums. Doxing is a reality. Yesterday I saw a job posting on a subreddit and decided to investigate who was offering it. Despite the throwaway I had the twitters, facebooks, blog posts and google plus profiles of every single person involved in the project within five minutes. And those are guys on the Internet - when a limo driver creamed me across four lanes of the 405 two months ago I looked his ass up online. Dude doesn't exist. His interaction with the Internet is a test pattern. Just the basic stuff that you find when someone doesn't know about Spokeo and its ilk.
The majority of people on the Internet don't understand the Internet. They think they're shouting through their car windshield. They don't understand that they're breaking through the bedroom window and inviting everyone for miles around to do the same. I don't know what effect that's having on society but it isn't good, I reckon.