The drooping angle is called anhedral. As you would expect it has a destabilizing effect. You'd need to personally talk to the designers but on the big military cargo planes I believe it was done to counter some of the high amount of stability provided by the combination of the high mounted wing and the sweep angle. Another thought is that by drooping the wings slightly you lower the lift center more towards the true CG of the load itself so the "pendulum" effect of the heavy load doesn't have as much effect when climbing or diving. But that's just a guess on my part.