Got to page 2 and skipped to the end here, so I don't know if someone mentioned it already, but the reason the horde loses more often is due to the alliance advantage regarding what has been termed, since vanilla, the triangle of victory (or just the triangle depending on your realm).
The triangle is made up of the 3 minimum bases needed to win AB. In the alliance's case it is the stables, the lumber mill, and the blacksmith, which form a triangle on your map (hence the name). These 3 bases are optimal for the alliance due to proximity to the ally starting area; this is most noticeable at the LM, where an ally riding at top speed will beat a horde player, going to the same lengths, to the flag by about 2.5 seconds.
This is where the psychological comes in, assuming several others came with the 1st ally to cap the flag, the alliance now holds the top of the hill where as the horde should just be reaching the top; seeing all those allys on top of the hill generally causes the horde to pause for a moment. During said pause, some will decide to fight, and other will decide to run screaming down the hill... The resulting confusion will often result in the horde's 1st big loss of the BG, which in turn leads to people leaving while calling everyone else noobs.
The horde's version of the triangle is supposed to be GM, BS, Farm... However it has often been brought up that this combo is inferior to the alliance's triangle due to the GM being difficult to defend and taking so long to get out of due to being in a depression. By contrast, the LM is the high ground, and as such easier to defend for a variety of reasons... The fact that you can see the whole world from up there, is alone a huge advantage.
It should be noted, the BS can be reached by the horde more quickly than by the alliance, the horde can begin capping the flag as the allies come into casting range. This initial advantage can be nullified at the higher levels if a dk in the group uses path of frost, which gives the alliance a straight run at the bs flag.
