These attacks were in response to the FBI shutting down Megaupload.com:

The filesharing website, which allowed users to freely exchange large video and audio files, was closed overnight and its operators were charged with criminal copyright infringement. They are accused of deliberately ignoring requests from film and music firms to remove pirated material, while making more than $175m from membership fees and advertising.

Anonymous supporters attacked the websites of the Department of Justice, the FBI and Universal Music Group, among others. The hacktivists used a technique called a Distributed Denial of Service to overload their targets with web traffic and effectively force them offline.
http://www.telegraph.co.uk/technolog...oad-raids.html