cuz the internals are still moving, and when you put it in park its like putting it in park when your driving with the t case engaged.... does that make sense?
if your tcase is disengaged and you put it in drive, all the stuff in the tranny will be moving, so when you slam it in park its starts clicking. you have to go from drive to neutral, and wait then put it in park.... that should make a little better sense :P