I think it's been a long time in the making but the real spark that lit the fire was the media's (and general population's) passive treatment of Trump's lying in the 2016 election period. During that time he was already telling whoppers, and the media sort of humoured it. You can see now four year's later that his version of reality is deeply embedded in a way that is nearly impossible to correct for - this make take a generation now to pass. Simply calmly stating that his words are "without evidence" or "unsupported by facts" etc etc isn't enough. When figures of authority depart from objective reality you have to stop it right there. He should never have been given another interview question other than to question his lies. But that was perceived as partisan at the time so they just let these things slide by.