This is considered to have been retaliation by Taira clan which was at war with Kai-Genji since Kai-Genji was in partnership with Omi-Genji (Minamoto clan) during the battle of Omi and so on, or by other clout close to Taira clan.
近江攻防などで近江源氏と連携していた甲斐源氏に対する平氏かそれに近い勢力による報復行動と考えられる。