Malbim: He saw that all of Yisrael were pleased with David [and wanted to return him], but no one was doing anything. He saw that the Chachamim must do so. Ziknei Yehudah will be first, and the others will follow them.

Malbim: (a) To return him to his kingship. (b) To return him to his house.

Malbim: They should say that Chachmei Yehudah agree with all Yisrael to return David to his kingship; the debate is about who will be first to return him to his house. For this, Ziknei Yehudah must be first.

Rashi: This was part of the message to Ziknei Yehudah. Malbim - since all Yisrael want to return him to his kingship, you [Yehudah] should be first to return him to his house.

Radak (13): The 'narrator' tells that while David was commanding Tzadok and Evyasar, word came that Yisrael wanted to return him.