It occurs if there's a sustained mismatch between your vestibular sense and visual odometry. That's ok if the game only involves looking around in a fixed position, but if the avatar is walking/flying then you're likely to get VR sickness, no matter what the frame rate is.

Apparently, VR users do habituate to the effects after a few days, in the same manner as space sickness, but in the intervening period there can be some unpleasantness.