Well, I'm no expert on it but I think people don't quite get that there is a differance between Anonymous and the hordes of /B/ despite efforts being made to enlighten people on this here and there.
My immediate guess would be thatit's some internal drama that snowballed, Anonymous itself may not even be involved on the fringes. I don't follow the site so I can't make an educated guess.
That said like "Encylopedia Dramatica", "Something Awful", and other sites, "4-Chan" seems to have beeen attacked before it's all part of what goes on when sites irritate people and get attention, or have a lot of people constantly involved in one sort of drama or another.
I doubt we'll ever know what happened for sure.
One culprit I think people might not be considering are various goverments. I mean Anonymous *DID* declare war on most of the civilized world, and started taking down some computer systems. While I unlikely, given the difficulty of actually tracking down a "collective" like this and doing anything, it's possible the authorities, not understanding what they are dealing with, decided to try and make a counter-strike on what they saw as the center of activity or whatever. This is extremely unlikely compared to many other possiilities however.