Welcome!!!
Interesting problem!! Could be that the breakaway switch is defective. It may have been pulled accidentally at one point and the contacts of the switch could be burned and stuck in the operated position. There is no connection between the 120 V cord/power system and the brakes of a trailer. The breakaway system brakes are powered by the 12V battery in the trailer itself. No battery, no breakaway brake system. Could also be the plug wiring from the truck has a problem, and that should be checked out too.
Should you have to move it a short distance to a dealer for repair, and if the problem is truly the breakaway system, simply disconnect the trailer battery and that should release the brakes. You can see if they are really activated or not by using a compass near the wheels. If the brakes are activated, you should see a definite deflection of the compass needle.