John Gill Commentary 1 Kings 1:18

"And now, behold, Adonijah reigneth; and thou, my lord the king, knowest it not:" — 1 Kings 1:18 (ASV)

And now, behold, Adonijah reigns
Has set up himself as king, and is by some saluted as such; but lest David think she was suggesting he was guilty of the breach of his oath, or on any account to be blamed, she adds:

and now my lord, O king, you know [it] not ;
which as it acquitted him from all blame, so it made the sin of Adonijah the more heinous, that he should do this without consulting his father about it.

And was not only neglect of him as a father, and an act of disrespect and disobedience to him as such, but even of high treason, to assume the throne in his father's lifetime, without his consent.