The explanation is simple: drums. In particular, the Drums of Battle, which increases melee, ranged, and spell haste. In an exclusive interview with WoW Insider last March, Neg of Nihilum remarked that the one thing that impressed him (and presumably Nihilum and their raid planning) was the effectiveness of haste. Haste is a statistic that became extremely prevalent in Patch 2.4, with many new items containing haste, including spell (currently AWOL) haste gems. Our raid specialist Marcie Knox wrote thoroughly about haste in a series of articles under her column RaidRX. It's a lesson that the top raiding guilds seem to know by heart. Nihilum was so impressed by haste that most of their core raid members leveled Leatherworking to be able to use drums, because it benefits the raid more than the individual unlike other professions like, say, Tailoring.
Before Patch 2.4 dropped, many members of the game's top guilds furiously leveled Leatherworking. With almost all raid members carrying the Drums of Battle, a raid can have an almost permanent haste buff that stacks with Heroism/Bloodlust. Having all your raid members level Leatherworking -- and basically for one item -- for raid progress is a masterstroke that shows the lengths that hardcore raiders will go to push the envelope. Clearly, it isn't the only reason Nihilum beat the Eredar Twins, but with a 6-minute enrage timer breathing down on every raid group doing the encounter, it certainly helps.