As Oz mentioned, it doesn't take much to push a person into endorphin overload and hit subspace, subsequently loosing any muscular control or ability to be coherent enough to respond to stimuli. Remember, it can take a few hours to a few days to come down from an endorphin high. So having another scene shortly after you've already had one could push you past your ability to manage the experience, and your brain would just turn you off and reset when the chemicals are back to normal.