I think it comes from the same thing that makes me feel sick playing FPS games. On the elliptical, my legs are telling me I should be moving in a way that doesn't align with the visual input, and the combination makes me feel sick. For some video games, my vision says I should be moving like I'm walking, but my body is sitting still. It's not really motion sickness (I don't really get motion sick on a ship, for example, and I can comfortably read in a moving car) but more a kind of sensory clashing that makes me feel like I'm going to throw up.
I get that big time from playing FPS games. In that case it's complete lack-of-motion sickness. Body is completely stationary while eyes and ears say the complete opposite.
I cannot read in a car or on a bus due to motion sickness (yet I can on a train or plane as those modes of transport go essentially in a straight line and don't throw my body about). So as long as my body isn't being thrown around at odds to input from eyes and ears my brain doesn't question anything.