Looking at the pictures the zombies have different colored eyes.. one even looks like he has two different colored eyes. It's possible that the blue ones are controlled by Richtofen but the others I am not sure of who they are controlled by, but it looks like some of them are focused on different directions. They are not all going after the same target.
Doh! Forgot about the "The lights of their eyes" achivement describing it as, "In TranZit, pacify at least 10 zombies with 1 EMP." So maybe the different colored looking eyes and the reason some of them seem to be focused on something else is because of a EMP?