It was in retaliation for Megaupload, as was the concurrent attack on Justice.org." Both sites are still down, and more attacks are being mounted; Anonymous members have now targeted the sites of the Motion Picture Association of America and the White House, and other government sites.
The whole process is part of what the group has charmingly dubbed "OpDonkeyPunch." They are even singling out the Democrats in Congress who support SOPA, attacking their websites and are also planning to target the Facebook and Twitter accounts of SOPA supporters.
For those unfamiliar with Anonymous; the fractured group of hackers has made a habit of using Denial of Service attacks as a form of protest. They earlier took out the site of credit card companies like Visa over Wikileaks issues. But they've also shown themselves to be vigorous supporters of free speech and democracy having formed Free Iran and participated in the "Arab Spring" events that led to the liberation of countries like Egypt and Libya.