It began when the media realized they COULD bring down a government, as they did with Nixon during Watergate, which came nowhere near the minimum requirements for impeachment.
Then, they began to realize that they had the power to influence public opinion, until finally, in 2008, they were able to install their own federal government, in Obama. The problem is that they see themselves as a part of the government, and as such will do anything to protect it, even if it means hiding not just incompetence and malfeasance, but HUGE and SERIOUS crimes from the public.
Disagree respectfully. This has not been an organic evolution within Western journalism, but an induced, organized rot introduced into Western high society, elite academia, and opinion-leadership journalism and infotainment professions by the Soviet intelligence services and the Communist International (COMINTERN) going all the way back to Feliks Dzherzhinsky and his Cheka. It will have been a Chekist who started the infamous Cambridge recruitment effort which eventually yielded the Philby Ring and put one of that ring next to the Queen herself (Anthony Blount).
In America, one of the first universities attacked was Columbia University, and another was Indiana University (home of Alfred Kinsey and his perverted phreak show). By the 1930's both campuses were known as Little Red Schoolhouses. Dzherzhinsky and his successsors also attacked Hollywood (Dalton Trumbo, Will Geer, and dozens of others) and tried to turn it into a closed shop of Red propagandists -- they were frustrated by Ronald Reagan, and the Red unionists were beaten by George Meany of the AFL-CIO and Walter Reuther of the UAW.s Unfortunately, no champion arose in education, journalism, or academia to confront them, and their career of destruction and disinformation (they invented that word -- dezinformatsiya in Russian) has been virtually unchecked.
Others will disagree, but I think the press and "media" have been in Soviet hands for a long, long time -- at least since the late 60's, when Vance Packard wrote about the emerging monochromatic commentariat in media.