Tpoic has been brought up a few times. These are the reasons that I remember individuals putting out there.
1) Hard Braking. The G-Force is so intense that it takes the shifting foot (left foot) off the peg. The reason the right foot does not come off is because there is more of a concious effort to keep it there because at that level, they use the rear brake.
2) Quicker Turn In. Approaching a turn at their pace, some riders can put more of their weight off the side of the bike and forward by shifting their weight with the leg and foot forward. Again, you will only see this on left turns because of the right foot still apllying the rear brake.
3) Stretching. Once their shifting is complete, this is the only time they really have to stretch the foot since it is constantly going through the gear box.
Those are three I consistently hear. My guess... #2. So with that, there is four cents worth of opinion and I have been told plenty of times I am full of b.s. so I attribute this post to my increasing post count to get out of junior status.