Amazon’s official statement.
LJ user claims responsibility. Amusingly, I found “Weev’s” community posting (which I must admit looks pretty unconvincing, and the WSJ blogger isn’t particularly convinced by it either) not via WSJ, but via links on LJ. But the general theory (elaborated further here) that it was the result of some party or parties (maybe Weev, who knows?) gaming the user-rating system does seem plausible, since a few people had noticed books getting delisted for no particular reason back in February but it didn’t become a widespread phenomenon until Easter weekend. More plausible than, say, that Amazon intentionally delisted all GLBT titles over a holiday weekend as a policy decision and then backpedaled on it, anyway.