Radak: He understood that David wants to fight him, and he thought that it is very unlikely that he will be saved from him. ¹ Kli Yakar - if David will die, Yishai will blame Eli'av for not stopping him. It is upon him to rebuke him, for one must honor his older brother! He pretended not to know David's intent to fight Galyus, and rebuked only for coming and abandoning the flock; David will realize, all the more so for wanting to fight, and David will express his intent!

Kli Yakar (citing Mahari): He did not see what David brought [on his father's command], and assumed that he came on his own to see the war, and abandoned the flock. Malbim - for coming to the war, he said "Lamah Zeh Yaradta

Kli Yakar: Now that you left them alone, they will be diminished!

Targum Yonasan: Bakranusach. Radak - Beis and Pei are interchangeable. This is a matter of Hefker and lightheadedness, to forfeit your life and abandon the flock. Also in Shoftim (9:4) he translated "Pochazim" as Bakrin, which is like Pokrin. Also a Mishnah (Pe'ah 6:1) says 'Hevker' in place of Hefker. (Also refer to Shmuel 2 10:16:2:3.)

Kli Yakar: You think that war is pleasurable, and you envied us, therefore you came. We are in pain, and you suspect us!