Speaking of loose nuts......
I had the exact same thing happen with my 17 foot bayliner right by Grand Coulee Dam except the nut did fall off. I was pulling it behind my old dually with a camper on the back and NORMALLY could not see the boat back there ever. All of a sudden, I saw some sparks, and then saw the boat on the left side of the truck which was really not normal. Crazy as it seems, I waited until it came behind the truck and then lightly touched the brakes and it kissed the back of the camper bumper. I stopped on the side of the road with no damage to the boat or trailer, and only cosmetic damage to the camper bumper. We were lucky.....scary is an understatement to say the least. And you know what sucks....is that I tried to pry that SOB ball off about two months prior and nothing would budge that sucker not matter what. Now, I weld every ball on there I have.....it is worth it in my opinion.