Trailer brakes

I bought a new plug to hook up to the boat trailer and when i was hooking it up i noticed on the old one the blue for the brakes was hooked up for back up not on the brakes. So on the new one i hooked it up correctly so the blue is on the electric brake connection. When i picked up the boat from the shop and i got home as soon as i started to back up the brakes locked. I have no idea why so i recconected the blue to the back up and it worked no problem. I'm not sure what is going on. Does anyone have any good tips or ideas on this issue?