This is the first time in history that South Park was unable to meet its deadline in its 240+ episode history. All because of a power outage. Comedy Central said:
“On Tuesday night, South Park Studios lost power. From animation to rendering to editing and sound, all of their computers were down for hours, and they were unable to finish episode 1704 Goth Kids 3: Dawn of the Posers in time for air tonight.”
There's a ridiculous amount of work that goes in to making each episode, and when a power outage creeps on to South Park, it'd obviously screw everything up.

[Deadline]