Essentially, what you need to know is that GB want everyone to get to the Mall at the same time, so that they can finish in a bunch sprint, which Cav has the best chance of winning. Other teams know this, and have tried to get away from the main bunch (peleton) and keep a lead all the way back.
But that is very hard to do, as it is much easier to ride in a bunch of 60 riders than a bunch of twenty, especially when a lot of those twenty have used up energy on acceleration and lone rides trying to get away.