Worked like this since the system started. What happenings is a group is pulled to the same shard. The quest against all odds is best done in group so people generally join a raid to get onto a single shard. This shard is now flooded with alliance and made a priority for horde moving then onto it. The horde generally playing the game solo see a massive wall of enemies and leave making the shard even more lopsided.
It happens to the alliance as well. It's why I don't get the point behind against all odds or kill x player in y zones quests it actively breaks the system they have.
As for people complaining about pvp op should see the timeless isle it's the content that had blizzard remove text from friend requests because people would use it for death threats.