Hackers are targeting the Church of Scientology because they forced YouTube into taking down the video of Tom Cruise:

A copyright violation claim by the Church of Scientology against the posting of one of its videos to YouTube has prompted a full-on assault by a group calling itself Anonymous.The video, in which Tom Cruise proclaims, in part, that Scientologists are the only experts on the mind, was pulled by YouTube over the weekend at the request of the Church of Scientology as part of a long-standing effort to keep copyrighted material from appearing on the Internet. Other sites have since posted the Cruise video in full.In response to the take-down of the Cruise video, a group of vigilantes–calling themselves Anonymous, or Anon–have retaliated against what they consider to be Internet censorship. The group includes computer experts capable of Internet mischief. In recent days, local chapter sites for the Church of Scientology have been defaced, and in some cases denial of service attacks have also prevented access to the same sites. Real-world attacks have included fax-spamming those same offices.

The hackers posted a video to explain their actions (a transcript of the video is here):