[G-V_Driver]

In my experience, the only thing that sets the brakes on the trailer is when the plug is pulled by tension on the safety wire. If the trailer is hooked to the tow vechicle and the plug is inadvertently pulled (by a kink or snag on the rigging, or by somebody who accidentally or intentionally puts enough pressure to disconnect it) the brakes will set and the trailer will suddenly become harder to pull until some combination of brake overheating or tire wear causes something even more interesting to happen.

Another reason to be sure you have plenty of slack in the wire and to check the setup any time you have been away from the unit while it is hooked up. It's also not a bad idea to gently tug on the safety wire during the hook up process to be sure it is securely attached to the receptacle on the trailer.