This is what the average WSG match on my battlegroup looks like at the end:
How does this happen? Well, the entire Alliance team except for me gets intercepted by about 6 Horde players while the rest picks up the flag, then all of them proceed to camp the Alliance graveyard for another 20 minutes (including their flag carrier).
I kill the DK + Mage duo and some other guy and then watch stealthed as the Horde team gleefully jumps around on the graveyard, two-shotting the Alliance players that keep on respawning.
As it turns out, the reason why those ~6 Alliance kept on respawning and getting themselves killed over and over was that they did not know how to avoid the rez. I'm not kidding. I've had people who managed to farm over 40 deaths while someone desperately tried to explain to them that they need to jump off the platform.
Although that level of inanity partly explains why the Alliance seems to lose about 90-95% of random BGs, I'd like to focus on the other team, the ones farming them:
It seems to me that most, if not almost all players, derive a large portion of their enjoyment of PvP from the illusion that they have deprived someone else of that very same emotion. If this is just a passive reaction to a perception, it would be Schadenfreude. But it's not just that. It seems like they really go out of their way to harass their opponents and try to kill them in the most aggravating way possible. Take the Mage and DK duo at the top of the Killing Blows list for instance. They were destroyed by a single Feral Druid just returning from a ~5 month WoW pause shortly before they commenced their graveyard camping. They aren't good players. So doesn't it make sense that they'd exploit a situation that allows them to delude themselves into thinking the opposite? Might that be the explanation?
Another plausible thought is the obvious one, relative anonymity. I mean, translate the same situation into, say, a soccer match. Imagine two teams playing soccer against one another. One team is clearly stronger, so much that they can pretty much do whatever they want without risking losing the game. So they just bunch up in front of the other team's goal, taunt them and gloat for 89 minutes while passing the ball to each other, only scoring the 1-0 right before the time limit. It seems perfectly absurd, but it's the rule, not the exception in PvP. I guess it's because of mainly two things: you don't really suffer adverse consequences from behaving antisocially and you don't feel much empathy for your opponents because you don't necessarily perceive them as fellow primates.
One last thing I've noticed from actually talking to people showing this kind of behaviour (we ended up on VoIP to organize a certain event) is that they seem to completely lack any understanding whatsoever of what I just described. If you ask them whether they'd rather like to win a challenging and fair 1v1 fight or whether they'd prefer ambushing someone currently engaged in such a match; they don't understand the question. It's like they don't even perceive the difference.
PS: I'm not, in any way, suggesting that all Alliance players are idiots or that all Horde players are sadists. These are just examples.