It originally started with the 'family the groom married into' uncles of the TAIRA no Masakado, and uncles of TAIRA no Masakado built a structure in Kanto region especially in Hitachi, Kazusa, Shimousa, and Musashi Provinces, and there was a trace that the conflict of interest between 'places where the grooms married' led to the struggle between TAIRA no Masakado, his uncle, and cousins.
そもそもの発端は、平将門の叔父達の「婿入り先」であって、それによって平将門の叔父達は関東、特に常陸国、上総国、下総国、武蔵国などに地盤を築いたと見られ、その「婿入り先」同士の利害対立が、平将門と叔父、従兄弟同士の抗争に結びついていった形跡がある。