I heard it too when I watched the episode. I think it just got borrowed a stock sound effect pool. That, or someone in Studio B was owed a favor by someone from Valve.
 
Whatever it is, it’s probably the same reason I’ve heard the Krazoa Palace music from Starfox Adventures on a show with Bear Grylls eating a few bugs, with no logical connection between the two. I’ve also heard music from Metroid Prime Hunters on Cops, appropriately enough the chase music from that game.

@PikaRobo  
https://www.youtube.com/watch?v=ChXzaZCsPIs

http://wiki.teamfortress.com/w/images/4/44/Halloween_boss_defeated_fx.wav
 
Here’s what HHH sounds like defeated, now I just need Sombra’s audio.

The headless horseless horseman would do great with the headless horsemanless horse from scootaloo’s episode.