The mighty Bhishma was separated from your army and became ferocious in that battlefield as an elephant becomes dangerous when its caravan missed and he is left alone. (All warriors from rival party, ran away leaving him alone because they could not bear more blows given by Bhishma.)
ते वध्यमाना भीष्मेण प्रजहुस्तं महाबलम्॥ स्वयूथादिव ते यूथान्मुक्तं भूमिषु दारुणम्।