The group began recommending the software after the take down of Megaupload in late January. Anonymous members DDoS'd UMG, the U.S. DOJ, the FBI, RIAA, MPAA and many others in retaliation of the raid.
Because the software is so easy to use, Symantec believes thousands of people opened themselves up to the trojan: "Not only will supporters be breaking the law by participating in DoS attacks on Anonymous hacktivism targets, but may also be at risk of having their online banking and email credentials stolen. The joining of malicious financial and identity fraud malware, Anonymous hacktivism objectives, and Anonymous supporter deception is a dangerous development for the online world."
More info here: Symantec Blog