User blog comment:DeathGodPrayer/Talk About What You Want!/@comment-82.178.241.162-20140115025012/@comment-1359322-20140115031015

Not much of a reason, the Vassago Clan and the Astaroth Clan had good ties with each other, then the Astaroth clan met their downfall. Vassago misunderstood the situation and thought that Rias and her team was responsible for the Astaroth's downfall. She learned the truth in the end though.