I saw my mother over the weekend and she asked the "Where are they coming from?" question. The best I could come up with is that when you get zombiefied maybe your olfactory senses get hyped up like animals and then when you have a bunch of living people in a stationary spot they get a general direction from the odors and aromas. And then maybe other zombies can sense when other zombies pick up a lead so even if they aren't in range of the source they might be in range of the other zombies which leads to a clustering. Mind you, I was just trying to figure some rationality, not that it was the best. Of course, my mom also says sometimes "you just have to squint" as she put s it when you're watching shows to enjoy them which isn't bad advice either.