Zombies are the liberal nightmare. Here you have the masses, whom you would love to love, appearing at your front door w/ their faces falling off; and you're trying to be as humane as you possibly can, but they are, after all, eating the cat. And the fear of mass activity, of mindlessness on a national scale, underlies the fear of zombies.
We live in zombie times. Fear of the mob. of the other, or the "rest of the world" (or of the red or blue states) losing their goddamm minds and pulling the few among us left...pulling us from all sides in to the pit of madness. As the nation begins to polarize and the "other side" is seen as increasingly dangerous and difficult to understand, a world in which humanity itsef dies only to rise up and tear the flesh from our very limbs seems less like fantasy and more like allegory.
That's not all of it, but I thought it was interesting. Go read The Walking Dead!