Although much is understood, a complete understanding of how this happens is not known. If you are reading in a moving vehicle, your eyes may be fixated on the page you are reading telling the brain you are still. However, your inner ear detects the ups and downs and turns of the journey. The brain receives this confusing message and sickness is the result. Fortunately, there are conservative approaches to avoid and treat this nausea.
Avoidance comes best from some common sense decisions.
If you are predisposed to nausea, avoid reading during rough moments in the journey. Try to choose a position where your eyes will be cued to the motion. In a car you are better off being in the front seat where there is more window exposure. On a ship, sit on the top deck where you are aware of the horizon. On a train ride sit next to the window.