NPC stands for "Non Playable Character." It's a gaming term for characters that you don't control that only add to the scenery and have repetitive meaningless things to say like, "Hi! Would you like a fish today?" A couple of years ago somebody on the chans speculated that there were more humans than there were souls to go around, so some humans may be NPCs.

The meme was kind of a background thing, applied to mindless following and cliche - prone folks, and SJW's (Social Justice Warriors) fit the bill, also.

Apparently, recently, some SJW decided to find something completely NEW to be offended by, and complained that the NPC meme was dehumanizing and therefore offensive.

Hillarity has ensued! It's the most fun I've had on the internet in a while!