I've played on a lot of vanilla servers and I've noticed this routinely across all servers. Noticed a few reasons that could likely explain this phenominon. I've thought about this a little bit. Even if you switched paladins and shamans on both factions the result will be similar.
#1 People roll horde to pvp.
Most people think the horde racials are better for pvp in vanilla so they roll horde for them. If you look at the racials outright this is easy to believe. Horde racials tend to outshine alliance racials in more contexts then Alliance racials do. If you look at horde racials also, they are also better in solo contexts or small group contexts then they are in group contexts. I do believe, however, that alliance racials are better suited for super top tier group pvp combat, but this view is not shared by the vast majority of the vanilla community. Thus, pvpers roll horde to pvp, primarily seeking an edge.
#2 Red vs Blue phenominon
Game theory explains why in most situations where there is a red color vs a blue color in competition all other factors being equal the red color will win:
https://www.youtube.com/watch?v=X31K6jammH0
However, in group competitions such as MOBAS (or alliance premades for example) blue is more likely to encompass strategy, and long term results, and wins more often. Red is humanly associalted with impulse (solo / small queues) If you look at the horde characters they also tend to look much more intimidating then alliance ones do. If you mix the color red, along with the general scariness factor horde characters have, the psychology behind the game gives horde a distinct SOLO advantage. I believe that horde players are more likely to assert agression because of their team color and characters features then alliance players are. If a gnome kills a horde player he is likely to come to a thought: "that little gnome shit." However, if a tauren or an orc defeats a gnome the gnome is more likely to think: "I have no chance in this competition." If you combine players of both factions at medium-low range skill this psychology has a huge impact.
#3 There are always many more horde players pvping then there are alliance players
Again, since most horde pvpers roll horde looking for that advantage, there are more of them. This means horde queues are always routinely longer then alliance queues even when there might be a 50/50 split in population
#4 Alliance premades are a response to a need while horde premades are not
Since most pvpers tend to roll horde to pvp, and since there is a distinct psychological advantage to being horde, and since there are always more horde queing then alliance are queing, Alliance premades always result. This gives strong willed alliance a chance to overcome some of the psychological disadvantages they might face. Facing a premade is scary for any pug. This also gives them a chance to filter out those super casual non-pvpers that heavily populate the alliance faction. Finally, it facilitates group play which is much more common on the alliance faction.
In conclusion: Those that pick horde to pvp are in the norm and are above average. Those that pick alliance to pvp knowing all of the above are the elite.
