Pranks... Prrrrrraaaaaaannnksssss...
The fantastic thing about this prank is that it works on multiple levels. If you happen to think for a moment that you've encountered an actual herd of reanimated corpses, then that's obviously going to be terrifying. But that's only nominally scarier than happening upon a group of living humans who enjoy dressing up as cadavers and frightening people for fun.
But probably the biggest horror is if you know enough to suspect that these zombies are improv dorks who were hired for an Internet prank. I literally have chills at the thought of all the fliers for sketch shows and invitations to get interviewed for podcasts you'd find yourself buried under.
Just look at the hideous improv nerdery on display in this making-of video: