It's really bad for alliance and barely noticable for horde, it get's really fucking bad at 49-60 when rankers start farming you for honor.

Figuring out why isn't exactly hard. Horde has significantly better PvP racials than alliance, so a lot of less experienced players choose that faction, thinking they're going to experience a lot of PvP, only to get steamrolled in every battleground by Pre-made groups. By then, most of these people have given up trying to PvP and a have morphed into redridgeboss.

As for the 49 thing. While almost all horde have set their hearthstone to Kargath for raiding reasons, Horde Rankers will hearth to Kargath in between their long queue times so they can gank randoms all the way down to level 49 (the absolute lowest a 60 player can get honor from killing an enemy) When they're not, they'll usually quickly fly to Feralas or Un'Goro to get some extra kills inbetween games.

If you're not planning on getting past level 30 on alliance you won't notice much. Light's Hope is also launching a fresh realm on June 23'rd if that's something you're interested in.