Albert Barnes Commentary 2 Kings 9:3

"Then take the vial of oil, and pour it on his head, and say, Thus saith Jehovah, I have anointed thee king over Israel. Then open the door, and flee, and tarry not." — 2 Kings 9:3 (ASV)

Flee, and do not delay - The probable purpose of these directions was both to prevent questioning and to make the event more striking.