It all relies on manipulating the queue timing (I always argue it falls under the definition of ‘exploit’ since it’s literally exploiting a weakness in the system), with a pre-organized group of players larger than the 5-man limit, to bypass the 5-man premade limitation. (‘group’ refers to a group of people, not the ‘group’ as in a WoW group of 2-5 players.)
However, this trick is only possible when the player pool in the matchmaking on your faction is small. The larger the player pool of potentially matchmade players it is, the more you’d need to compensate by “throwing bodies” at the queue to increase chances of getting people that are part of the pre-organized group to get into the same BG to essentially achieve the result of having a premade larger than 5 people in the same BG. (Which isn’t likely to happen since you’d be organizing several times more than just 40 players and it’d be a lottery who would get into an epic BG with enough players from the pre-organized group of people.)
The quicker the queue is, the easier it is to pull off this trick as well. Because when the queue is fast, it means it’s emptying out the player pool in the matchmaking for your faction at that speed. If it’s slow, it builds up and creates priorities depending on time in queue and other variables.
We saw people methodically abuse this in Classic AV when it first came, so much that it even prompted a response by Blizzard. Although Blizzard kept it politically correct and vague and basically said it’s not the way it’s intended, and due to the amount of complaints it was stirring up in both NA and EU, it led to a change to the matchmaking for Classic AV.
What they did was make the queue artificially longer, to give it a chance to build up more players thus making this exploit more difficult to pull off. But they underestimated the level of organization behind the premades, and the problem didn’t really go away. So they did a few other things I don’t remember exactly, but they got rid of most of the premades after a couple of months (yes, it took them months to accomplish their goal).
Anyway, so the point is that the queues are artificially made small by the faction split. Used to include the region split too, but I don’t know if there’s any region priority left in the variables anymore, and the regions are definitely not split anymore.
So when one faction has many players, and the other has few players, it means it’ll always be easier to pull off on the side that has few players since odds are their player pool will much more often than not have so few players that it becomes highly likely that a pre-organized premade exploit is possible to pull off somewhat reliably.
This exploit used to be handled by addons, but Blizzard disabled the addons in the API. First time they disabled an addon for it was in MoP iirc, but addons have popped up since then that tried to accomplish somewhat the same things and they’ve been disabled one after another each time since then.