It also happens when the air pressure drops causing the safety lock to pop that hold back the emergency brake system.that or you have a air leak between the tractor and trailer or you do not have enough(governor is not working or set properly) air pressure from the system to hold back the s cam preventing the emergency brake system from coming into effect. In other words the air keep the brakes from locking up as long as the air pressure is at the level/psi to do so.when you lose pressure the brakes apply automatically. Unless you ahve porr;y maintained brakes on each wheel then you may actually need new pads or to adjust the tension/pressure on the pads.just like adjusting brakes shoes on older model cars.just some of my opinions/ideas.
