Originally Posted by Wild Card
Most likely the pin adapter came loose. The 4 set screws. Mine has.
Could probably toss out to lack of maint on your end.
My toyhauler has 3800# pin weight not a single blemish. 15000miles.
No, it was correctly torqued, and didn't come loose. If the set pins come loose you're likely to have issues with the pin adapter rotating horizontally around the pin during braking or acceleration. The bent pin box base plate is a known issue with the pin adapter. Andersen has spacers to put under the pin adapter to distribute the load and prevent this and simply say it happens with some pin boxes because they aren't strong enough. I've personally seen the same issue on two other trailers now as I tend to talk to folks in the campgrounds when I see them with this hitch.
Go back out and look at your pin adapter and it will become quite evident that while there are 4 set screws, only two support the weight of the front of the trailer when hooked up. The 4 set screws surround the pin, and pull the adapter away from the pin box, with the two mount screws hitting the lip on the pin to stop the pulling away movement. The end result is the pin adapter supported a quarter inch or so off the pin box base plate, and the ball socket hangs out in the air so to speak. When you put the ball in the socket, the weight will have a pivoting type force on the pin adapter, attempting to rotate on the two set screws between the ball and the pin. The mount bolt opposite the pin prevents the rotation, with the end result being all the weight of the pin transferred to the two set screws in between the between the ball and the pin. Again, both Lippert and Andersen were aware of the problem when I spoke with them. As for your trailer, If it has a standard pin box there is a good chance it has a Lippert 1621HD pin box which is quite common in the medium weight trailers around 16k or so. Hit a bump wrong, get hard on the brakes, or just many miles of bouncing and your pin box will probably start to give as it was designed to have the weight distributed across the base plate.