Albert Barnes Commentary 1 Kings 1:7

"And he conferred with Joab the son of Zeruiah, and with Abiathar the priest: and they following Adonijah helped him." — 1 Kings 1:7 (ASV)

Joab’s defection on this occasion, after his faithful adherence to David during the troubles caused by Absalom (2 Samuel 18:2–17), may be accounted for by his fear that Solomon would be a man of rest (1 Chronicles 22:9) and by his preference for the character of Adonijah. He may also have thought that Adonijah, as the eldest son (1 Kings 1:5), had almost a right to succeed.

Abiathar’s defection is still more surprising than Joab’s. For his history, see the note on 1 Samuel 22:20. Until now, David and he had been the firmest of friends. It has been conjectured that he had grown jealous of Zadok and feared being supplanted by him.