Why does it say that all Ziknei Yisrael came? Verse 1 said that all Shivtei Yisrael came!
Radak: Our verse means that the Chachamim already came first to David to make a Bris, lest he punish them for supporting Ish Boshes for two years, and delaying five years before coming to make David king. After the Bris he sent them [to bring] Shivtei Yisrael, and they came and anointed him.
Who are the Zekenim?
Malbim: They are the Sanhedrin. They must authorize appointment of a king.
Why did they anoint David? Shmuel already anointed him (Shmuel I, 16:13), and also Yehudah (2:4)!
Radak: Divrei ha'Yamim I, 11:3 says that they anointed him to be king over Yisrael like Hashem said through Shmuel. Shmuel anointed him over all of Yisrael, just like he anointed Sha'ul, but it was not fulfilled until today. This is why David fought the Yevusi in Tziyon only after this.
Why does it say "Lifnei Hashem"?
Radak #1: The Shechinah dwells wherever all or most of Yisrael are.
Radak #2: They said that Hashem should be a witness between us in this Bris that we make.
