It is said that this was a plot of the Fujiwara clan who were aiming to the position of the Sadaijin since there was a tradition of an Imperial Family member not being allowed to become a Minister.
これは、皇族は大臣とならない当時の慣例に目を付けて、左大臣の地位を狙った藤原氏の陰謀と言われている。