In a blog post, CloudFlare's Matthew Prince notes that while this was a particularly large attack, Spamhaus is a frequent target. "Spammers aren't always the most lovable of individuals and Spamhaus has been threatened, sued, and DDoSed regularly," Prince writes. "Spamhaus's blocklists are distributed via DNS and there is a long list of volunteer organizations that mirror their DNS infrastructure in order to ensure it is resilient to attacks. The website, however, was unreachable."
A Pastebin post that appeared around the time of the attacks called Spamhaus "an offshore criminal network of tax circumventing self declared Internet terrorists pretending to be 'spam' fighters," and was signed with the Anonymous-esque "We are legion. We never forget. Spamhaus should have expected us."
Still, according to Softpedia's Eduard Kovacs, Spamhaus issued the following statement in response: "The DDoS attack carried out against the Spamhaus Web site over the weekend was carried out by a Russian criminal malware gang and not by Anonymous."