They own the system, they could just shadowban bullies' votes. They could have just done it on videos that were statistical outliers, too - raised a "bullied" flag.
It'd be pretty easy to isolate different voting mobs, too, and group them together. You could use them to just ban the mob that is targeting the video, and let members of other mobs' votes still slip through. I might even inform the user if I were cocky about the system: "You have been prevented from voting on this video because your account has been associated with a mass-downvoting group from reddit.com/r/ronpopeillives. If you feel you have received this message in error, send an email to appeals@devnull.youtube.com."
It'd be pretty easy to isolate different voting mobs, too, and group them together. You could use them to just ban the mob that is targeting the video, and let members of other mobs' votes still slip through. I might even inform the user if I were cocky about the system: "You have been prevented from voting on this video because your account has been associated with a mass-downvoting group from reddit.com/r/ronpopeillives. If you feel you have received this message in error, send an email to appeals@devnull.youtube.com."