It depends on the track most times. If there is not a good push lane where the trucks can turn around and grab another car quickly, they will put them all on the track first, then start to push. Not many pavement tracks have enough push trucks, let alone a good push lane. It may take as long to push all the cars on the track, then fire them off, but it saves a bit of fuel, so no one is fighting to be the last in line. Many dirt tracks will do this also, I guess we just do not see it as often cause the more sprint car friendly tracks have enough trucks.