1) Stability is highest with the load nearest the ground in line with the mass of the Load Truck
figure 2 figure 3
2)As the weight is lifted, the centre of gravity moves toward the load centre because the spread of the load, although still equally distributed has to balance between the centre of gravity of the truck and the centre of gravity of the load. What does happen though is that the higher the load goes the potential for over balance becomes greater.
It can be seen that as the weight is lifted, the angle from the pivot point reduces and so the load remains balanced. But any even a small amount of tilt results in the load being drastically further away from the balance point (front axle).
We have already established that the relationship between centre of load and distance from the pivot point has a direct relationship to the stability. Therefore say a 5% tilt forward at 2m lift may only move the load forward of the pivot point 10". Where as if the same load is 20m lift the same degree of tilt will send the load forward 100" and the distance x weight ratio is exaggerated directly by this amount.
The resulting force being toward the load. This will overcome the dead weight of the truck and result in over turning of the truck and load.