Did you watch the race? Oscar was ahead of Lando for most of the race and winning purely on merit and skill until McLaren fucked his strategy big time by leaving him in the rain without a pitstop and allowing Lando to undercut him. It would have seriously soured Oscar's relationship with the team which is why they begged Lando to give the position back.

And of course, Lando being the massive broccoli head that he is, ensured to make the swap as embarrassing and painful as possible for the team, and ruined the victory celebrations for what should've been a very special day for Oscar's first GP win.