Albert Barnes Commentary 2 Samuel 18:9

"And Absalom chanced to meet the servants of David. And Absalom was riding upon his mule, and the mule went under the thick boughs of a great oak, and his head caught hold of the oak, and he was taken up between heaven and earth; and the mule that was under him went on." — 2 Samuel 18:9 (ASV)

It would seem that the two things in which his vainglory boasted—the royal mule and the magnificent head of hair by which he was caught in the “oak” (or rather, a terebinth or turpentine tree)—both contributed to his untimely death.