Abhimanyu's untimely death is attributed to his enmity with Lord Krishna in his previous birth.

Abhimanyu in his previous birth was known as Abhikasura a demon. He was a friend of King Kans who left no stone unturned to kill his nephew Krishna. After Krishna killed Kans, Abhikasura vowed to take revenge on the former. Krishna, sensing the moves of the demon, wove a magic around Abhikasura, transformed him into an insect and captured him in a box.

When Krishna's sister Subhadra, after her wedding with Arjuna, accidentally opened the box, the insect got into her womb. Abhikasura was then born as Abhimanyu.

Krishna knew this and he rather sadly, prepared the ground for the death of his nephew and finally got him entrapped in Padmavyuha.
When Arjuna was mourning the death of his son and squarely blamed Krishna for it, it was then Krishna explained Abhikasura's reincarnation as Abhimanyu and the necessity for him, as the Supreme Power, to incinerate the evil force.

There is another story in which Abhimanyu is believed to be the reincarnation of Varchas, the son of the Moon God. When the Moon God was asked to let his son incarnate himself on earth by the other devas, he made a pact that his son will only remain on earth for 16 years as he could not bear to be separated from him. Abhimanyu was 16 years old when he died in the battle.