The Jester's intentions are explained in detail on his blog, in a post dating from September. In the post, the Jester describes the flaws of Wikileaks' "insurance policy," which consisted of making available an encrypted file, supposedly containing the secret documents that were leaked afterward. The file is useless without a decryption key, which would be provided by Wikileaks in case someone takes the site down.
The Jester hints of having obtained some sensitive information about Wikileaks itself, but said he decided not to upload it on Wikileaks as he doesn't believe the information would see the light of day. He then provides an encrypted file of his own, claiming the information is contained within -- again, as "insurance."
While it is entirely possible for one experienced and resourceful hacker to take down a site -- even a fairly large one -- by a DDoS attack on his own, it's not easy to prove whether the Jester is really behind the attack and, if he is, whether he was working on his own or if he had help. We're sure the story will only give more fuel to the various rumors that are flying around Wikileaks being taken down.