"smarter" zombies seem to be lured to the area by somebody (I'm thinking the Governor, or a new minion of his). It remains to be seen if they're really smarter. However, the zombies in the first few episodes of Season 1 were pretty smart - Morgan's wife twisted the door handle, one of them picked up a rock to try and break a window, and another climbed a fence. However, they dumbed them back down in Seasons 2 and 3.