Unity - Object stops moving when colliding with player

Question

This problem is a bit hard to explain and maybe confusing, but I will try my best to explain it. I use iTween for the movement of the cubes in my game. When the player collides with the activating cube / is a child of the activating cube, the cube starts moving. However, when my player jumps on a moving cube, the cube will stop moving for a couple of seconds and I don't know why. It has something to do with iTween.MoveTo, because this doesn't occurs when using iTween.MoveBy (like I did at Cube (1) 2 in the script below), but I have to use MoveTo.

Script:

using UnityEngine;
using System.Collections;

public class CubeMovement: MonoBehaviour
{   

public int startX; 
public float startTime;
public int pingPongX;
public float pingPongTime;
private bool started;
public GameObject playerObj;
public float originalY;
public float fallDownTime = 1912;

public GameObject activatingCube;

void Awake()
{
    pingPongTime = (startTime / (Mathf.Abs(gameObject.transform.position.x - startX))) * (startX - pingPongX);
}

void Update() 
{
    Player player = playerObj.GetComponent<Player> ();
    //All blocks start moving after 1 click
    //if (player.clicks == 1 && started == false) 
    //{
    //  StartX ();
    //}
    Debug.Log(started);
    //This block starts moving after player collides with activatingCube
    if (started == false && playerObj.transform.parent == activatingCube.transform) 
    {
        StartX ();
        started = true;
        Debug.Log ("Going to start");
    }
}

void StartX()
{
    started = true;
    Debug.Log ("Started");
    if (gameObject.transform.name == "Cube (1)2") //omdat dit blokje hetzelfde is als activating cube werkte het alleen met moveby
    {
        iTween.MoveBy (gameObject, iTween.Hash ("x", startX, "time", startTime, "easeType", "easeInOutSine", "loopType", "none", "delay", 0));
    }
    StartCoroutine(FallDown(fallDownTime));
    StartCoroutine (StartPingPong (startTime));
    if (gameObject.transform.name != "Cube (1)2") 
    {
        iTween.MoveTo (gameObject, iTween.Hash ("x", startX, "time", startTime, "easeType", "easeInOutSine", "loopType", "none", "delay", 0));
    }
}




IEnumerator StartPingPong(float startTime)
{
    yield return new WaitForSeconds (startTime);
    iTween.MoveTo(gameObject, iTween.Hash("x", pingPongX, "time", pingPongTime, "easeType", "easeInOutSine", "loopType", "pingPong", "delay", 0));
}
}

How it looks like:

Btw when the player collides with a cube, it will become a child of it, until it jumps off the cube. The cube stops moving when the player becomes a child of it and after a couple of seconds it starts moving again, why?