1 Sensory conflict theory
The sensory conflict theory states that motion sickness can happen if the person's perception of self-motion conflicts with another sensory input. In other words, when different senses report different information to the brain about motion, sickness can happen.
2 Low refresh rate
If the headset uses a lower refresh rate of on screen images than the brain's processing speed then sickness might occur.
3 Poor animation
When the animation is poor, the brain expects to see something but it sees a different thing. This can also lead to sickness.
4 Not adapting to head movement
If the graphics the person sees don't adapt properly to head movement then the person might get sick. This happens because the brain expects something, but finds another. (See Is Virtual Reality Bad for Your Eyes?)
5 Disagreement with inner ear
If the data reaching the brain through the inner ear is different than the data reaching the brain through the visual sense, then sickness might happen.
6 Theory of postural instability
According to this theory, motion sickness can happen because of poor postural adaptations in response to an unusual visual stimuli. (See How Virtual Reality (VR) affects the eyesight?)
7 Your brain thinks you got poisoned
According to one theory, the sickness happens because the brain believes that you got poisoned and this is why you are getting mixed signals.
8 Mismatching acceleration data
One of the causes of sickness is that the brain gets conflicting information about acceleration from different senses due to the fact that the motion that happens in the game doesn't really happen in real life.