Radak #1: All nobles of the Pelishtim stood in one counsel to come to fight against David. They are called Melachim and Rozenim, even though everywhere they are called only Sarnei Pelishtim, for amidst pride they considered themselves to be kings.

Radak #2: Perhaps kings of other nations were with the Pelishtim.

Malbim: Sometimes rebellion is due to the king, like Tzidkiyah who rebelled against Nebuchadnetzar, and the nation was drawn after him. Sometimes it is due to the nation, which rebels against a king that rules over them, and their king joins them. This rebellion was from the people ("Lamah Ragshu Goyim") and also from their kings ("Yisyatzevu Malchei Eretz v'Roznim." The people rebel from one side, and the kings on the other side with the nobles of counsel.

Rashi: They took counsel. Radak - an expression of Yesod is used, for counsel is to action like the foundation to a building.

Radak: They knew that David ruled based on Hashem; Shmuel, Hashem's Navi, anointed him to be king.

Malbim (1-2): They came "Al Hashem" - to cast off the yoke of the seven Mitzvos that David forced them to fulfill, "v'Al Meshicho" - to cast off his yoke and his service.