What kind? How old?  Something has to be telling the brakes to come on, usually is the brake controller............although it could be a short in the plug?

Like Chad said, something has to be activating the brakes. If it isn't an errant controller, it has to be another source of voltage going to the brake wiring.

You can try disconnecting the activating wire from the controller to the trailer. If the trouble persists, the problem probably isn't the controller, rather it's the wiring. If the problem goes away, it's probably the controller that is at fault.

Gard