Anonymous is the result of overreacting officials. When someone goes to say something that they don't want traced back to them, they post as "anonymous" in the sense of the true meaning of the word. The problem is that when lots of defamation posts were being made anonymously on various sites, some officials decided that it was a group, not just a bunch of random people posting as anonymous internet users. Rather than understanding that many forums that allow guests (non-members) to post will show their name as being "Anonymous", they figured that this was an actual name that was being used by a group.
Eventually, once it started hitting the media that there was apparently this cyber-hate-group known as Anonymous, the members of the 4chan forum started taking advantage of that by publishing promotional material, like the headless guy in a business suit. People also started attending protests wearing the "V for Vendetta" mask.