While no one has stated that, it's evident that the factions weren't made up of random people. There's no reason why everyone would like to play with their buddies. This is absurd.
While there was a dominating EU Horde guild and the list of bosses who were first killed in vanilla indicates that there were 7 Horde to 40 world first alliance kills. It wasn't even close to me, as a member of a world-first-pushed guild. Although great players could beat vanilla hordes but it was much more easy to become an alliance player than it was before.