What's up with my Trailer???

Well I picked up the boat today and brought it home (Moomba XLV - GG) and I try to reverse....the wheels lock up. For some reason the emergency breaks on the trailer is locking up. So I try again and again, I'm about to ram the front of the trailer through my truck. I'm pretty pissed. This by the way was at the gas station about to fill up. Well I fill up and drive it to the marina to try again. Sure enough it happens still. Then I try backing up really slowly and it works and I can back up and put it in the water.

Take the boat out and try to back up in my drive way - LOCKS UP AGAIN! So I went ahead let all the brake fluid out of the braks and then it works. WHAT THE HECK IS WRONG WITH MY TRAILER?? Am I missing something here?