That is a very good question, and actually part of the answer lies in the Question.
There is something that all humans have,
for some reason this is not common knowledge.
This thing I call a Construct exists on the astral plane,
it is, you could almost say, a shadow of the person.
Actually what it is is a creation made of all the energy from all the attention that has gone to define that person over their lifetimes.
As you know the Law of Manifestation say's that where you put thought and emotion...
if it is maintained,
something will eventually manifest.
Well think of all the thoughts and feelings you have had about yourself...
and all the thoughts and feelings others have had about you....
just in this lifetime alone,
that is a lot of manifestation energy.
Well this creates this Astral Shadow that has a lot of the attributes of an actual soul...
except no self motivation....
it's totally created upon outside attention,
so it is limited to "what was".
Anyway when a person dies the place they pass through is the astral plane and one of the major laws there is ...well, we'll talk about 2 laws.
One is that we draw to ourselves that which resonates at a similar frequency....
and another is ,
where we put our attention...
we will go.
So normally when we pass to the InBetween Time as I call it,
we sort of have a forward looking direction....
we are heading in a certain direction,
quite motivated by our souls desire "get on with it". ("It" being evolving)
However there are some souls who are so invested in their past lives,
in certain aspects of the life they just left.....
that their resonance rather than the usual desire to evolve....
is the desire for what they were.
This resonance ...as a way of speaking....
turns their heads towards their construct (the definition and energy of who they were)...
and simply by making this ...choice.....
it pulls their construct into them....
not the usual scenario...
and they become trapped so to speak by their astral construct,
and can not move forward,
because they are trapped in yesterdays definition...
I have some actual .... experience in this area....
unfortunately, that cost me about 20 years of this life.
I used to do something that for lack of a better term I call, Yoga of the Death State. Basically my teachers taught me how to die,
without actually cutting the silver cord.
This made it possible for me to die countless times,
and experience what death is like....
so I learned a lot of Rules of how things operate in the InBetween time.
Well, this will probably sound strange to most folks. But something happened in my life where, against my Teachers Yells, he he...I went out there and "turned" to face my construct. The intention was to "go back to who I used to be"... I know, sounds insane... actually you can read about it here in more detail, just scroll down to "The Humpty Dumpty Episode"....
http://mysticorbtalismans.blogspot.c...ss-closet.html
Anyway, since I hadn't severed the cord, I wasn't trapped in an astral realm,
but ....as you can read in the post on the link....
I got seriously messed up.....
So to make this concise.
The shadow people are souls who clung to their constructs at death,
and are temporarily trapped in a reflection of who they were,
and the world they knew.