My parallaxing is moving by it self?

I'm trying to follow a tutorial on youtube but my parallaxing is moving by itself even if I don't move. Here is the code, the tutorial uses a orthographic camera.

 using UnityEngine;
 using System.Collections;
 
 public class Parallaxing : MonoBehaviour {
 
     public Transform[] backgrounds;            //Array (List) of all the backgrounds and foregraounds to be parallaxed
     private float[] parallaxScales;            //The proportions of the camera's movement to move the backgrounds by
     public float smoothing = 1f;            //How smooth the parallax is going to be. Make sure to set this above 0
 
     private Transform cam;                    //Reference to the main Camera's tranform
     private Vector3 previousCamPos;            //The Position of the camera in previous frame
 
 
     //is called before start(). Great for referencing
     void Awake () {
         //set up the camera reference
         cam = Camera.main.transform;
     }
 
     //Use this for initializing
     void Start () {
         //The previous frame had the current frame's camera position
         previousCamPos = cam.position;
 
         //Assaigning corresponding parallaxScales
         parallaxScales = new float[backgrounds.Length];
         for (int i = 0; i < backgrounds.Length; i++) {
             parallaxScales[i] = backgrounds[i].position.z*-1;
         }
     }
     //Update is called once per frame
     void Update () {
         //For each Background
         for (int i = 0; i < backgrounds.Length; i++) {
             //The parallax is the opposite of the camera movement because the previous frame multiplied by the scale
             float parallax = (previousCamPos.x = cam.position.x) * parallaxScales[i];
 
             //set a target x position which is the current postion plus the parallax
             float backgroundTargetPosX = backgrounds[i].position.x + parallax;
 
             // create a target postion which is the backgrounds current position with it's target x postion
                 Vector3 backgroundTargetPos = new Vector3 (backgroundTargetPosX, backgrounds[i].position.y, backgrounds[i].position.z);
                 //fade between current position and the target position using lerp
             backgrounds[i].position = Vector3.Lerp (backgrounds[i].position, backgroundTargetPos, smoothing * Time.deltaTime);
 
         }
         //set the previous camera position to the camera's position at the end of the frame
         previousCamPos = cam.position;
     }
 }