Until I can establish otherwise, I date the break point at 1964, when Henry Luce gave up his position as Editor-in-Chief of Time, Inc. He had sufficient conservative influence within the media industry to leaven the tendency of journalists to lurch leftward.
http://www.timemediakit.com/us/timemagazine/press/bios/luce.html
So far my research shows no other event or series of events that would explain it. IIRC, LS was working on a book that would establish about that time frame as when the media shed all pretense of being “objective,” if ever they were.
Update: our research did NOT show what I thought-—a sudden lurch during or after JFK. Rather, it showed a very steady but consistent move further left by ALL major papers every year. The implications are that you cannot tie this leftward lurch to a specific event, but rather to larger forces that are more difficult to quantify. It is across the board-—At. Constitution, Cleveland Plain Dealer, NYTimes, WaPo, and LA Times moved at about the same rate.
A historian's study of that issue is much to be desired. Larry's A Patriot's History of the United States, good as it is, omits any mention of the telegraph that I was able to find. He said that the book had to be ruthlessly cut down to fit within the covers of what his publisher was willing to print . . .IMHO that is like omitting any mention of radio and TV in a discussion of politics in the 20th Century.