Unity randomly resetting gameobjects position to world zero

I have a problem with some gameobjects randomly resetting their position to (0,0,0). Their position is correct during Awake and Start but in the first Update the position is reset. If I try to move the object, the position is immediately back to (0,0,0). This don't happen everytime I run the scene. Sometimes it might work dozens of times in a row and sometimes the bug appears many times in a row.

The objects have a SpriteRenderer, Rigidbody2D, PolygonCollider2D and a script attached to them. I don't move the position in the script so that should not be the problem. There is not errors shown in the console so I really have no clue what could cause this random behaviour. I'm using Unity 5.1.2 64-bit on a windows 7 machine. I built the game for android and the bug randomly appears also on android devices.

Here is the code I have in the script in case it helps.

     public Transform activationHeight;
 
     private Transform copterTransform;
     private Transform _transform;
     private Rigidbody2D _rigidbody;
     private bool inAction;
 
     void Awake() {
         Debug.Log ("Awake position: " + transform.position);
     }
 
     // Use this for initialization
     void Start () {
         Debug.Log ("Start position 1: " + transform.position);
         _transform = transform;
         _rigidbody = GetComponent<Rigidbody2D>();
         copterTransform = GameObject.Find("Copter").transform;
         Debug.Log ("Start position 2: " + _transform.position);
     }
     
     // Update is called once per frame
     void Update () {
         Debug.Log ("Position: " + _transform.position);
         if (copterTransform.position.y > activationHeight.position.y && inAction == false) {
             inAction = true;
             Debug.Log ("Activate");
             _transform.position = copterTransform.position + Vector3.right * 20 - activationHeight.localPosition;
             _rigidbody.velocity = -Vector3.right * 20;
         }
         if (inAction == true) { 
         
         }
     }
     void OnTriggerEnter2D(Collider2D other) {
         Copter copter = other.GetComponent<Copter>();
         if (copter != null)
             copter.Detonate();
     }