Artist's comment which explains it a bit more:
JP: レイマリがエグめの喧嘩して「博麗の巫女様」「霧雨のお嬢さん」と呼ぶ回
TR: The episode where ReiMari gets into a nasty fight and call each other "Hakurei Shrine Maiden-sama" and "Kirisame Ojou-san"

Another one which is already in english:
Commenter: "looks like they gonna fight"
Artsit: "because they called by names that they didn't want to be called by each other…"