The bug was ostensibly called by combining two abilities together: Rushing Jade Wind and Spinning Crane Kick. The former gives the latter a multiplier, which is supposed to be around 30%, but appears to have had at least a couple of extra zeros accidentally added to the end. As can be seen from the header image, windwalkers were posting incredibly high DPS when using these two abilities together. This wasn't an isolated issue, there were various reports on the forums, as well as videos. Another example can be seen to the right, from Wowmartiean, in the raid finder.
WoW Insider readers will no doubt recall the previous bug with retribution paladins, where Censure had a similar modifier problem, resulting, again, in some disproportionately high numbers. As we mentioned in our recent patch 5.2 PvP round-up, windwalkers were due some buffs, but this seemed a step too far! Blizzard agreed, and for clarity, this is no longer in the game. It remains to be seen whether any action will be taken against anyone who is considered to have exploited this bug, but it has appeared and been hotfixed so quickly that, with any luck, it will not have had any major impact. As Blizzard noted on twitter, they ran rolling restarts to repair this within a very short time of its occurrence, and PvP Developer Brian Holinka noted that a fix was underway shortly earlier.