There are two theories for this; one theory is that Emishi tried to settle a puppet emperor to gain ascendancy, and the other is that Emishi tried to prevent Yamashiro no Oe no O, who was a descendant of the Soga clan, from taking over the emperor's throne so that friction with other major clans would be avoided.
これには蝦夷が権勢を振るうための傀儡にしようとした言う説と他の有力豪族との摩擦を避けるために蘇我氏の血を引く山背大兄皇子を回避したと言う説がある。