Saul was afraid of David, because Yahweh was with him, and was departed from Saul.
Procopius Gazaeus interprets it of the Holy Ghost, whose grace was vouchsafed unto him: he might be afraid in his melancholy fits, that as he had attempted to take away the life of David, that David would contrive and seek an opportunity, and take away his life, and seize the kingdom which God had given him, and his being with him strengthened these fears: