No, it's not just due to rendering lag. Motion sickness is caused by a disparity between the what the eyes perceive as motion and the vestibular system of the inner ear [0].
That's not all, though. Another major problem with VR is the lack of compensation for vergence [1] and accommodation [2]. These two systems are critical for depth perception and when they are not accounted for the result is confusion of the visual system, eye strain, fatigue, and even nausea.