One theory holds that because the system of birthright inheritance by the eldest son had not been established at that time, the conflict broke out because Yoshimasa's property was arbitrarily divided between his brothers (Masakado's uncles) TAIRA no Kunika and TAIRA no Yoshikane; another theory, based on the "Masakado Ki" (Record of Masakado), holds that the conflict broke out either over a daughter of the former Kokushi (provincial governors) of Hitachi Province, MINAMOTO no Mamoru, or the daughter of Yoshikane; and a third theory that states that the conflict broke out because of the interference of MINAMOTO no Mamoru and TAIRA no Maki in a territorial feud.
長子相続制度の確立していない当時、良将の遺領は伯父の平国香（國香）や平良兼に独断で分割されていたため争いが始まったとされている説、『将門記』などによる、常陸国（茨城県）前国司の源護の娘、或いは良兼の娘を巡り争いが始まったとする説、源護と平真樹の領地争いへの介入によって争いが始まったとする説などがある。