At some point during the primaries, they must have figured out there was no stopping him.
The Conservative Treehouse reports:
Confirmed: Rupert Murdoch Instructed Fox News To Take Down Donald Trump August 2015…
In an extensive article within New York Magazine, mostly outlining the rather sordid details of Roger Ailes, readers may also note specific confirmation of something we outlined in August of last year (2015). Fox News owner Rupert Murdoch instructed Fox News executives to take down Donald Trump.