It was based on the background that, back in those days, if a deer, which was regarded as a divine messenger, was found dead in front of someone's house by officials, it constituted an offense whereby they would face a serious charge or be punished by a fine of three mon (an old currency unit in Japan).
その理由は当時、家の前で神使とされているシカが死んでいる所を役人に見つかると罪となり、重罪または3文(通貨単位)の罰金となった。