Short version:
Inner Ear says you are moving, eyes say you are still (since all you can see is the interior of the vehicle). The when the eye and inner ear tell different things, this means to the old part of our brain that something is wrong, and for most of human history when that disconnect occurred it was because we ate bad food. So we developed a certain reaction, to remove the bad food before we died from it.
Result - motion sickness leads humans to blow chunks.