I honestly think it began subconsciously, not that a bunch of journalists went to school and decided to slant the news a certain way upon graduation but rather it happened because:At this point I'm not sure how anyone can believe it's not deliberate - the pattern is well established.
a) mostly people of the same bent went into journalism (not all but most)
b) they weren't exposed to ANY contrary ideas or if they were, they dismissed all of them as cranks
c) they then so desired the approval of their peers (e.g. Pulitzers and Emmys) they slanted more
d) their reporting DID show them the underclass of American society and they were imbued with the human desire to help change it if they could.
On this last one - I'm referring here to the same thing that happens when a right-wing Southerner graduates from law school and gets out into the real world practicing law - MANY of them are hardcore supporters of the death penalty (for example) until they see it's much easier to execute or jail an innocent person than they realized. So in that sense as the late Jack Germond says, what happens is not so much that every journalist is a liberal but they start their careers (in most cases) reporting on things like local corrupt cops or homelessness or things that all of us should feel can be addressed, changed, and rectified. Just human decency, liberal or conservative.
That - of course - does not explain what happened with the rapid decline of Biden that they refused to address.