回 in 怎么回事 is seen as a measure word by some. There is however linguistic evidence suggesting otherwise. For instance, (1) 回事 can be preceded by a measure word, at least in some dialects or vernacular uses: 咋个回事, 哪门回事. 兩碼回事. (2) It behaves differently from other noun phrases containing the measure word "回": 一回戏, 哪一回戏, 什么一回戏 or 怎么一回戏 一回书, 哪一回书, 什么一回书 or 怎么一回书 一回事, 哪一回事,什么一回事 or 怎么一回事 (3) 回事 does not usually mean 一件事 (e.g., 别把自个太当回事了!). To me, 到底是怎麼回事/什麼回事 ("what exactly has happened" 你们一见就吵,到底是怎麼/什麼回事) is different from 到底是什麼事 ("what exactly is the matter" 你说有事找我,到底是什麼事). What's your opinion on this?