The sum of the moments (think torque) initially did not create rotation about the fixed points of the crane-truck (the tires). If you notice the crane arm is pointed out at an angle...so the distance between the crane and the truck was not as great as it was when the crane arm is parallel with the ground.
When the crane arm was parallel the distance was the greatest, creating the greatest moment about the crane-truck tires. It obviously overcame the moment about the tires due to the weight of the crane-truck causing rotation.