One theory from the believer side (believer as in I believe that many of these events are a real glimpse of the afterlife), comes from mediumship communications with afterlife entities. Many of these entities claim that most people go through a period of deep rest upon physical death, or when our silver cords are severed from our bodies, which can vary from a few days to many years. There apparently are spiritual hospitals where these unconscious spirits are taken care of until they eventually awaken to their new enviroment. I don’t have strong evidence to support my argument here, but it is one take about this issue from the believer side. This is also the scenerio that most secular dualists and Spiritualists believe occurs as well.
On a quick note here, it helps to have at least somewhat of an open mind before we pass on from what I’ve read about this. Unlike our matter system, or dimension, mind has much more power on the ‘other side’. Our thoughts can very well be our reality. If we believe very strongly that we do not survive physical death, we may very well stay in a very deep coma type sleep, and for many years potentially from what I’ve read.
I’ve also read that many people who have very strong doubts about our minds surviving physical death may deal with another possibilty, not realizing that they indeed are ‘dead’ and may make many futile attempts to get the attention of others, to no avail, until help from the higher spheres attempts to rescue them. NDE’s are as varied as our minds are, so it doesn’t surprise me that there are so many differences between one person’s experience compared to another.