It's pretty certain that Awareness during Sleep Paralysis, or what you are calling RISP, is a phenomenon of REM sleep. Stress, lack of sleep, jet lag and other factors increase the frequency of ASP experiences. It may be some years before researchers can adequately explain ASP or the reason why some people have it much more often than others. Meanwhile, distressed experiencers may deal with despite the lack of an explanation. Paradoxically, the key is to learn to relax while in the grip of Awareness during Sleep Paralysis.
Some people actually enjoy the sensation of bodily immobility.
Some even seek to induce it.