Why hip will move upward and downward automatically when start to drag slider at x,y and z axis?

Hi, I am beginner in UI slider Unity 5. I am facing the problem of hip position will move upward and backward when I just start to drag the slider. Code used:

public void OnDrag(PointerEventData data) { _translation.x = _sliderX.value; _translation.y = _sliderY.value; _translation.z = _sliderZ.value;

     StartCoroutine (translation ());
 }
 IEnumerator translation()
 {
     Quaternion _rotation = Quaternion.Euler (_hipRotation.x,_hipRotation.y,_hipRotation.z);
     Matrix4x4 _m = Matrix4x4.TRS (_translation,_rotation,_hipScale);
     _newPos = _m.MultiplyPoint3x4 (_oriPos);
     hip.gameObject.transform.position = _newPos;

     yield return 0;
 }

Hope someone can help me because I am facing this problem for many days already. Thank you.