I'll get a little bit controversal here;
On Ax3, Froome attacked and never looked back. Never bothered with the radio, never reduced his cadence. Right after the stage, questioned are asked. VAM, average power output, total time etc. all these performance-related datas are calculated and compared to those known dopers. Time and wattage of Froome was amongst the best of those known dopers.
Today, he and Porte again dropped the rest of the group. Only this time Froome was constantly talking to Portal on the radio. After his initial enormous attack on Contador, he slowed down a bit and rode with Nieve for some time. Then he past him, talked to radio and caught Quintana. He tried two strong attack but Nairo answered back. Froome was able to drop Nairo earlier but he didn't, instead he tried two attacks and stopped. Only when it was close enough, he did the final attack that ensured him the stage win.
Now. Here is the controversy, theory, speculation, you name it. Considering the time gaps he achieved on Porte and Valverde at Ax3, Sky didn't want to cause that much gap.
I say, Froome was talking to Portal about time. The two were trying to optimize the attack moment in order to provide enough time gap for the stage win but also slow enough to reduce the "suspicious talk".