The Puranas have it that Jaya-Vijaya were cursed to be born as demons on the Earth for three births. Thus, they were, at first, born as Hiranyaksha and Hiranyakashipu. The Lord killed the younger one (Hiranyaksha) first.

In their second birth as Ravana and Kumbakarna, it was Kumbakarna, the younger one, who was slain first by the Lord.

They were Shishupala and Dantavaktra in their third birth. The Lord feared that the demons may avenge Him in the same manner by attempting to kill the younger among his siblings first. Hence, in order to protect Adi Sesha from any trouble caused by the demons, the Lord Himself took birth as the younger one!