The trim system (compensator) is connected to the air brake system. When you deploy the airbrake, the aircraft wants to pitch the nose up. So, the trim system trims the nose down to compensate.
After you deploy airbrakes, you have to make sure to return the airbrake button on your joystick to the centre (neutral) position or the trim system will continue to trim the nose down. The same is true when retracting the airbrake except the trim system will continue to trim the nose up.
This is how it works in the real aircraft.
When you open the emergency trim (compensator) guard, the link between airbrake and trim is temporarily broken.