Either because they thought the right of the crown was his; or rather, from secret grudges, because they perceived themselves neglected by David, and possibly by Solomon too; and from carnal policy, that they might secure and advance their own interest, which they saw to be in manifest danger.
Related Commentaries of 1 Kings 1:7
1 Kings 1:7
7 And he conferred with Joab the son of Zeruiah, and with Abiathar the priest: and they following Adonijah helped him.