I think some people are more prone to OBE (I projected spontaneously from about 35 years until I learned to induce) and I believe it has to do with my natural sleep tendenciees- it takes me a long time to go to sleep, while my hubby doesn't ever remember having one at all, but he's a fast sleeper- goes to sleep fast and sleeps very deeply.
I think it has something to do with catching the spontaneous exit simply because we're awake enough, and then 'knowing' it's happened. Awareness.