Rashi: It is improper for a son to sit next to his father. Normally, David was between Sha'ul and Yehonason. Now that David was missing, when Sha'ul sat Yehonason rose and did not sit until Avner sat in between. It does not say here that Yehonason sat, but we find that he rose later (verse 34). Malbim - Yehonason intentionally sat next to his father's place initially, so his father would recall David's absence.

Radak: Yehonason did not want to sit next to his father, lest his father get angry about David, and be able to hit him. ¹