Careless mistake. I don't blame Warren Beatty or Faye Dunaway (Beatty was clearly befuddled and while he was trying to figure out how to handle it best, he handed it off to Dunaway so she would know what happened and Dunaway, thinking he wanted her to say the winner, just read the film title), but you know Tom Hanks wouldn't have allowed this shit to go down. So from here on, AMPAS, Tom Hanks needs to be the guy who presents every category. He cuts down on unnecessary airtime and will give more time for the speeches.
They wouldn't make themselves that ridiculous on purpose. And they wouldn't want to make the producers of La La Land feel so bad for what would generally felt as a pretty silly joke.
Mistakes happen all the time. Wouldn't have thought that this would happen though. Good thing is: It shows how few people really know which movie wins. Otherwise it would been easier to fast righten the mistake.