The vertices around the lower thigh are not weighted to anything else but the thigh bones and I only have one armature modifier. I'm fairly certain it's reacting to that unmoving lower thigh bone but just don't know why. I don't even know what to google for this problem! I have used many many bone constraints on this rig but as far as I can tell no constraint should result in a mesh deformation without the bone actually moving. Does Blender just become glitchy when you use too many bone constraints?
 I also want to add that this problem is absent when I import the model and animation into Unreal Engine 4 so it's looking more and more like a bug or a Blender feature that I'm not aware of.