Well in previous DR games all they've really done is mill around and wait for you to come right up to them before they do anything with the occasional one moving for you, now though there is more mentality about them, they can see you from further away and react to you, their actions can have a domino effect too, if they start screaming 'rargh brains' or whatever, then others further out can hear them and move to see what's going on. Put it another way, in previous DR games, the zombies had a small sphere of action about 2 ft all around them, outside of that they never really knew what was happening, here though each individual zombie has a much much larger sphere, or perhaps not one at all, so they act much more like you would expect a zombie horde to act. I liken it to modern zombies compared to 70's zombies. In the classic zombie movies, zombies were just brainless eating machines that milled around until you got too close, the modern zombies in TWD etc are much more.....proactive, I guess, for lack of a better term.
“It's a movie. If you are going to believe and be affected by an action film, you shouldn't go to see Pocahontas because you are going to think you are a Disney princess” Chloe Grace Moretz, what a legend.