Lift spoilers and the airbrake are two different secondary flight controls on the Bae-146/AVRO. Obviously the lift spoilers are on top of the wing and the airbrake is the 'opening' arse.
However, when the hydraulics are pressurized the airbrake will be retracted and the lift spoilers down. The AVRO RJ has an automatic lift spoiler function and spoilers will come up when WOW.
But also if taxi speed is more than 33kts.
Maybe this was the case with this particulier aircraft.