The White House tried to move a reporter to the back of the press room, but she refused. Then Trump walked out.
White House Allegedly Interferes in Briefing Seating:
According to Paul Farhi of The Washington Post, a White House official ordered CNN White House Correspondent Kaitlan Collins "to give up her front-row seat and move to the back of the press room before President Trump’s briefing on Friday," contravening "a plan managed by the White House Correspondents’ Association and agreed to by White House officials last month." Although the official "suggested the matter would be resolved by the Secret Service" when Collins and the other reporter refused to move, no action was taken.