If I'm not mistaken it was popularized by the Fighting Game Community (FGC). There's a related subreddit (r/Kappa). PogChamp and Bible Thump (which are also on the list on the linked page) were also popularized by the FGC. The interesting thing is that in terms of viewers, Fighting games have a substantially less proportion of the total audience compared to MOBAs and FPSs. It's possible that FG viewers just spend much more time chatting than other spectators because the average Fighting Game match goes by in about 2 minutes.

Besides trolling, r/Kappa is also known for sponsoring many FG players to get to tournaments around the world, so you might come across players who list them as a sponsor (in the same way you'd maybe see someone sponsored by EG - Evil Geniuses).

In "twitch speak" the term can in some ways be used to indicate sarcasm, as replacement for "j/k", or just trolling in general.

eg. "This stream's quality is fantastic kappa!"