Tresses,
To answer your question, in my particular situation it was not really associated with any position. In my personal experience I think the event is associated with sleep paralysis. That being said, I think that's the physical catalyst or precipitant of the event but what ensues after the event starts I personally think is paranormal in nature.
In my instance I think it was related to sleep patterns, and stress at the time this was happening. I think that and perhaps some chemical imbalances at the time were what brought the conditions to play which started the sleep paralysis event. Purely physically speaking sleep paralysis happens when the sleep mechanisms of the brain kick in which paralyze body kick in yet our conscious brain still is fully awake. That brain paralyzes itself so that we don't go failing our arms and legs while we're sleeping.
I personally think (at least in my case) that this state is what leads to the "Out of body" style of event. Now, is there some other factor which is non-physical in nature that also causes the push into a sleep paralytic state? Good question, not one which I have an answer for.