I had a chance to catch up with Jason Jerald after he taught a class about Human-Centered Design for VR at the IEEE VR conference, and he explained each of the five different theories that may cause simulator sickness including the
Sensory Conflict Theory,
Evolutionary Theory,
Postural Instability Theory,
Rest Frame Hypothesis, and the
Eye Movement Theory.
Jason also talks about ways to mitigate simulator sickness including implementing different viewpoint control patterns, using a cockpit, providing a leading indicator of movements, walking in place, vibrating the floor, reducing texture complexity, and limiting optical flow in the periphery.
See the full story here: http://www.roadtovr.com/five-theories-motion-sickness-triggers-virtual-reality