When people subconsciously (or at times intentionally) form a significantly long line in an area next to much shorter lines for no apparent reason. Zombie lines are usually seen on the road (especially at intersections) involving drivers, but also apply to checkout lines in a store.
This action is similar to how zombies behave, one aimlessly following the other. Also see: lemming
I don't know why there are 20 cars in the middle lane when there are 5 cars in the left lane and 2 in the right lane. Definitely a zombie line.