Ittai was a Philistine from Gath. He was associated with Gibeah in Benjamin (2 Samuel 23:29; 1 Chronicles 11:31). The Philistines had set up a military post in Gibeah at the start of Saul's rule (1 Samuel 10:5; 13:3).
David was stepping aside for King Absalom, who had gained the loyalty of the people of Israel.