They think Motomichi started this incident in compliance with Yoshiie's thoughts in order for Yoritoshi to lose his position and power, because Yoshiie did not like the spread of MINAMOTO no Yoritoshi's influence in Mutsu Province.
陸奥に源頼俊が勢力を植えつけることを快く思わない義家の意を受け、頼俊の失脚を狙い事件を起したと見られている。