Jamieson, Fausset and Brown on 2 Samuel 21:15-22

15-22. Moreover the Philistines had yet war again with Israel--Although the Philistines had completely succumbed to the army of David, yet the appearance of any gigantic champions among them revived their courage and stirred them up to renewed inroads on the Hebrew territory. Four successive contests they provoked during the latter period of David's reign, in the first of which the king ran so imminent a risk of his life that he was no longer allowed to encounter the perils of the battlefield.