"The Trump administration’s criticisms of the CBO are unusual. Prior administrations, both Republican and Democratic, have steered clear of attacking the credibility of the agency, which many lawmakers regard as a neutral arbiter. The CBO’s current director, Keith Hall, was appointed by Republicans in 2015."

It seem obvious that the CBO does have something to say that President Alternative Facts and his Merry Gang do not want people to hear. Since they cannot silence the CBO, attacking their credibility seems to be the only viable alternative they have left to get things their way without major hassles.