Why did the Battle of Oshu occur where Yoritomo destroyed the Oshu Fujiwara clan, the real aim was not Yoshitsune

Even though they defeated Kiso Yoshinaka and destroyed the Taira family, Yoshitsune, who made Yoritomo angry, was driven into suicide in Oshu, and even the Oshu Fujiwara clan was destroyed... Such [Battle of Oshu] is often talked about as the end of the tragedy that Minamoto Yoshitsune follows. In some cases, it appears that even the Oshu Fujiwara clan was destroyed because of the plague god Yoshitsune. But is that really the case? If Yoshitsune…