That's because they believe that the mouth is the channel for a magic power which can harm themselves and their fellow tribesmen if it escapes.
因為他們相信，嘴巴是神秘魔力出入的孔道，予以暴露會同時危害自己與族人，這與我們現在所說的「病從口入、禍從口出」似乎不謀而合。