You will need to apply the proper Skin Weights so that portion of your mesh doesn't move with the wrong joints.
Click on your Left ankle and foot joints at the normal position, and go to the Skin > Edit Smooth Skin > Paint Skin Weights Tool.
The Paint Skin Weights tool tells the mesh where to deform with which joint. So I guarantee that if you click on that joint, you will see white on the wrong leg. Check it because with that little bit of movement, the color may not be a bright white, but a light gray or just enough so you don't notice it.
The best way I've found to clear up issues like these are to flood the area with black (no control), then gradually paint with white (full control) using the
Add, not the
Replace option. And don't paint where you can't see, like around corners, because you'll end up painting unwanted areas. Make sure you pan your camera around to see where you are painting.
More information on Paint Skin Weights