Jamieson, Fausset and Brown on 2 Samuel 18:9

9. Absalom met the servants of David--or was overtaken. "It is necessary to be continually on one's guard against the branches of trees; and when the hair is worn in large locks floating down the back, as was the case with a young man of the party to which I belonged, any thick boughs interposing in the path might easily dislodge a rider from his seat, and catch hold of his flowing hair" [Hartley]. Some, however, think that the sacred historian points not so much to the hair, as to the head of Absalom, which, being caught while running between two branches, was enclosed so firmly that he could not disengage himself from the hold, nor make use of his hands.

the mule that was under him went away--The Orientals, not having saddles as we do, do not sit so firmly on the beasts they ride. Absalom quitting his hold of the bridle, apparently to release himself when caught in the oak, the mule escaped.