Perahpes they are just waiting for us to leave our bodies, where we are more valnurable to the evil entities.
Some spirits are just waiting for a door to open so that they can come into our plaine.
I ave actulay had one episode without the terror and feeling of an evil pressance. there was one time when i had woken up in the morning and could not move whatso ever, i was only frustrated. then I saw the form of a person standing next to my bed, i could see through it, and it's aura was magnificant! it bent towards me and i heard a voice as if it were in my head say, i love you.
it's then hugged me.
I believe that sleep paralysis is a spiritual expirience. but i still agree that doctors are partly right about how it happenes, but i still believe that during SP we are close to the viel between the astral and physical plaines.