@ccaats- sorry to revive your 9 year old question, but I felt bad that no one ever replied and I have a little experience with this same phenomena so I figured I'd reply is case you were still active on the forums.
I'm not 100% sure, but I believe this is the result of the open door causing the Body Control Module (BCM) to wake up. When it does, you'll also notice that the audio system wakes as well- probably so that it's ready to make chime sounds, etc. If you wait about 30-45 seconds in a quiet area without pushing any buttons, opening or closing any doors, etc. you should hear a quiet "click" that sounds like it's coming from all the speakers at once. This signifies that the BCM has returned to "sleep" mode (which is a good thing, because my testing has found that the BCM draws between 2-4 amps when awake).
The way I discovered this was when I made some recent mods to use all LED brake and turn signals and the complex resistor and relay network I devised was accidentally leaking a 12v signal from one pin of the BCM to another, which was causing the BCM to constantly wake, sleep, wake, sleep, etc. and the end result was a dead battery the next day...
Anyways- that's my theory on the disc spinning up (my does it when I open the door too and I don't have the Nav unit, just the rear seat DVD).