I think that this is the first thing which would be felt when dealing with undead humans. The way zombies move in a not quite human, shambling way. The way that Draculas shadow had a mind of its own. Things like this would put observers in an uncomfortable state of mind and would breach the Uncanny Valley - Something which should be human acting in nearly - but not quite - human manners.
I like this explanation. I wonder if there's a way to portray this during a gaming session. Seems like a great way to run an undead-heavy campaign.
Just watched it last night so it's fresh in my mind, but it seems like the animatronics in Evil Dead 2 managed to harness that kind of creepiness when the possessed humans would shift from monstrous to almost human to fully human and back again.
Now, the undead that manages to breach the valley and come out on the other side will be something to be feared; because something evil which can make its victims naturally feel comfortable with its presence can end up doing the most depraved things to you when your guard is down.
Taken out of context, that describes anti-social behavior pretty well.