Sergio Perez was told two laps from the end of the sprint race that he could enter the pit lane and park his car in the garage. Halfway through the sprint race, he had fallen back to the rear due to a spin, which made Red Bull decide from a strategic point of view not to let the Mexican driver finish the race.
With Perez starting from the pit lane, it is permitted to make changes to the set-up of the RB16B. The parc fermé will no longer apply to the Mexican's car. Perez will be the last to start and so Red Bull wants to make some changes to the set-up of the car that should make overtaking easier.
The Milton Keynes based team will probably choose to run with less wing, which means a higher top speed and less downforce. That set-up seems to be working well for Mercedes, something Verstappen would have preferred himself.