Maybe Max is already gone? Could have been the last straw, Horner has to go, start from the bottom and build up a new team?
Otherwise surely Red Bull Racing is more attractive to Verstappen with Horner in charge? He knows how to create not just a winning team but a team that can completely dominate. That's got to be 10 times more important than a Team Principle getting along with your Dad, despite what the tabloids and netflix might want us to believe.
There was a group of people at Red Bull (Horner, Newey, Wheatly, Marshall, Marko and others) who are all as competitive as hell and for a long time they likely put their differences to one side because combined they ruthlessly created one of the most successful teams of all time. It has all been slowly crumbling since Dietrich Mateschitz passed away. Max's continued success has merely created a facade to cover what began to collapse 2.5 years ago.