IMO, the "lean" was already happening in 1960, when MSM "reporters" covered up JFK's promiscuous infidelities during the Presidential campaign. 1964, Daisy and LOLITS. 1976, Ford the Klutz. 1980, Reagan the Warmonger. 1992, GHWB and supermarket scanners. 1997 or 1998, pretending Monica's blue dress until it tested positive for Clinton's DNA. 2004-2006, Memogate and Cindy Sheehan. 2008-Present, Obamian lapdoggery. 2015-Present, steroidal TDS. Just a few milestones.
I think MSM lean became a crash and progressive collapse during Clinton's MalAdministration.