I'm thinking the breakaway switch is closed, probably the pin is not all the way seated. Try pulling the pin out, then replacing it and make sure it is all the way in. When that pin is pulled, it closes the switch, which directly connects the battery to the brakes, causing them to be engaged.
2015 Jay Flight 32RLDS Elite
2012 Chevy 2500HD Crew Cab LTZ (6.6L Duramax/Allison)
Equalizer Hitch (1200# bars)