Sleep Paralysis can have a physical or emotional base depending on the situation, and always links with the emotion "fear". To be paralyzed is to be in fear. To live in fear based on events in one's life is to create panic, anxiety, and paralysis where the person feels they have no where to go and things are closing in.
Have you ever woken up, unable to move, paralyzed by fear? There can be any number of reasons people suddenly awaken from sleep and feel physically paralyzed - but it all goes to fear - usually about something in your physical reality that you have no control over after which you carry that emotion into your dream time and back to physical reality.
There are many causes of sleep paralysis, physical and also not-so-physical, such as being awakened suddenly when your consciousness has not fully integrated back into your physical body. This often results in physical symptoms of nausea and disorientation - by coming back into the body too quickly. Remember that only part of our experience is in 3D Earth physical reality grid. There is so much more, interesting experiences simultaneously occurring in other grids realities (subroutines of the program.) You are much more than the person sitting in that chair reading this.