I know it's a meme and it's not used very frequently, but in your head in your head what's in your heeeeaaddddddddd?
Apparently it's pathing hell. Each zombie has a giant queue of move attack commands and when you get a bunch of corpses, it's a very heavy load of pathfinding requests all at once. Maybe they should only have one attack command at a time, or at least something more sensible?