The helicopter was slinging a hydro tower into position. After positioning the tower, the ground crew secured two of the support cables to their foundations. The pole was then shifted to accommodate securing of the remaining two support cables. As the two secured cables became taut, the pilot felt a bump and the helicopter became unstable. The pilot then released the load and was able to regain control of the helicopter. The helicopter then landed safely. There were no injuries and there was no damage to the helicopter.