It used to be that BG's had set mount speeds and no one could exceed it, as of late though I have seen a lot of players able to move alot faster mounted for example; at the start of AV or Isle of Conquest move out of the pack and separate from it by large margins. It is not class dependent so not just pallies kicking on their buff.
How is this done? Perhaps just the guild perk "Mount Up" at work?