Because Hamilton was leading the race with 8 laps to go and he was being asked to save fuel and given a delta time. He was told that Button had been given the same instruction yet Button was gaining on him in each sector. Obviously with a victory so close its naturally concerning for a driver to see his teammate pressuring when they have both been told to hold position. As soon as the two had duelled for the lead, Whitmarsh was seen giving firm instructions to both drivers that fuel was critical and under no circumstances were they to continue challenging each other and risk either an accident of running out of fuel. It was a necessary team order if you will.