I heard it has something to do with...
-the camera may not follow the character correctly
-camera may not be in sync with action
-rumbling in controller may not be in sync with camera, sound
-sounds/camera may not be in sync with camera/sounds
Apparently there's a bunch of factors like those that lead to sea sickness, because what your body sees, feels, hears, and senses (the inner ear liquid thing) all have to be in sync for you to function 100% properly. That, and sometimes a really shaky camera can just screw you up.