(360 at least) Beast Mode's left trigger doubles zombies.
Zap zombies while running around as beast, when they respawn, they are doubled.
It just happens constantly because zapping zombies is the best thing in the game, so you do it as you run around and you get more zombies.
Purposefully gather them and zap the crowd, run off, and double the zombies.
32 (nonbeast) kills on round 1 is my highest, but more could be made.
Weird note: this part won't give points, but as you turn human, any nearby zombies get blown away. These count as kills, even though the zombies respawn (to kill again, get even higher kill count).
If it is a glitch, either it is only on 360, or the community is blind to the fact that zombies double, or somehow you guys aren't zapping the zombies, which is mind boggling. Why play SoE if you don't Odin-spank the zombies?