Ok, here is my thought.
The axles if original are shot, the trailer rides rough. Weight might have been added in the very rear of the trailer creating a situation where the back of the trailer has been hammered down.
What you are going to find is that the trailer frame is bent down right where the second axle passes through a notch in the frame.
This happened to my trailer After I had repaired the separation issue and used the trailer before changing out the axles.
I fixed this for little more than the price of the axles by using 20' of 3" angle, a jack, and a welder...
1/2 Ton 4WD Truck, 72 Sovereign