I'm trying to picture this. Are you saying the lift arm cylinders fail because the operator curls the load with the bucket cylinders?
Not exactly as you describe. I am describing actions after the operator has released the loader control. The bucket cylinders are supporting the boom pole but now the operator is using a winch connected to the end of the boom pole which is increasing the forces on the bucket and lift arm cylinders.
With the back dragging operation, it is the movement of the tractor which is increasing the cylinder pressure.
What is happening in the scenario I described is that the cylinders become pumps. A force on the cylinder piston generates a hydraulic pressure which is trapped by the loader valve. Given the right leverage on the loader cylinder the pressure can go extremely high causing damage.
Tractor designers protect the 3 pt hitch by including two relief valves. One is in the circuit as you try and raise a load on the lift arms. This relief valve limits what can be lifted. Once the arms are raised and the control is in a neutral position this relief valve is no longer able to protect the 3 pt hitch.
Designers, anticipating shock loads as a tractor is driven with a raised heavy implement, include a second relief valve called a safety valve. It is directly plumbed into the 3 pt hydraulic cylinder and is always there to protect the 3 pt. Its setting pressure is much higher and is set using a fuel injector tester.
The loader valve has no such internal protection. Newer tractors can be ordered with a hydraulic accumulator connected to the loader arm cylinders. This acts like a shock absorber and smooths the tractor ride. After market versions called Soft Drive.
Imagine you lift a load with your loader that it can barely raise. Now, wanting to move more material you start shoveling more stuff into the bucket. You can continue adding material until something breaks as the hydraulic pressure will continue to increase to support the load.
Hope this all make sense.
Dave