The Horde of this battlegroup seemed content to ingore many of the core rules of playing AV and in general people followed the wrong orders. In almost every game the first thing out of someones mouth was "ZERG VANN". Sometimes it worked sure. But zerging requires a few things that were not always there.
1) You NEED a good tank. If you are going to all out zerg then you really need a geared tank in order to handle Vann and and four marshals. If you don't have a geared tank, in tank spec you won't zerg.
2) You will need MOST of your team up there. The problem with the encounter is that the marshals randomly charge and swing at a person before returning to the tank. With 4 marshals up people will die. You need enough healers and dps to compensate for those that end up getting killed. So at least 30 if not more need to be there in order for a zerg to work well. Maybe you can do it with less if they are all well geared but it is hard to find a solid group like that on my side these days. So for a zerg to work you somehow need to convince everyone to go to Vann.... which despite what people were always yelling... did not happen.
3) Stop for nothing. If you are going to zerg you have to go all the way. No one can stop for Towers, Graveyard, Belinda ... NOTHING. Your entire zerg must go straight from the cave to Vann with no stops in between. Breaking group coherency will break a zerg like nothing else.
Honestly though I think in many cases these things are to much to ask from a random group of people, at least Horde side on this Battlegroup.
So zerging is probably not a good idea for random AVs. Time and time again we won games with good hard Defense. In every game that we wiped then on Galv and started taking towers back, they did not recover and we won. Defense is the best possible way to win with a random group in AV.