I stated this earlier, either here or on a different Clinic thread.
Froome, from the looks of it, probably had a motor in his bike, the same one Armstrong and Cancellara used at different points in their career.
During the initial acceleration on Mont Ventoux, it seemed as if Froome couldn't keep up with the cadence, then settled down and got in sync with it.
The change in cadence was just too ridiculous to have been caused by anything else. And in all my years of watching cycling races, I've never seen such a thing. Only then and only with Froome.