Such nighttime anxiety is experienced by many children, and you are trying to find a rational explanantion for it. It is not rational; it is not based on actual experience of parentsal leaving or abandonment. It's not going to be useful now to discuss why she is behaving this way. The focus should be on consistent management of the behavior, particularly on consistently placing her back in her bed, with little or no discussion. If you are patient, supportive and consistent, you will see the behavior change over time. But it's 'bite the bullet' time - don't waver.
Kristy, Its going to be hard, but you will need to do the "supernanny" thing. Place her in bed, hugs and kisses (read a book) tell good nite and leave the room
If she gets up, tell its night time, and place her back in bed. Do this once more. Then if she gets up again don't say a word, just place her back in bed. this could happen for a while but she will stop and realize you mean business.
As for why she is doing this, could be she really does have some time a fear that you will leave or, she doesn't want to go to bed and she will talk and say what she can to keep you with her. Kids are very smart. Hope this helps