Either an axis / joystick button assigned to that function that you don't know about, or that axis sensitivity / null zone incorrectly defined.
For each of your controllers, under Options / Controls / "Buttons and Keys", sellect "All events" and browse across the middle and right columns to see what joystick buttons might be assigned, and disable those you do not need for that controller and can play tricks like the one you describe!
Then, on the "Calibration" tab, make sure that the speedbrake ( Spoiler I assume that's what you meant ) axis gets FULL sensitivity and full left ( 1 ) null zone.
If you were talking about the wheel brakes, apply the same method..., only in that case I have:
