Enemies disappear randomly

I have implemented various enemies in my game but for now, I'm confronted with a problem when I want to add pre-defined movement to all enemies.

Firstly, the code managing enemies worked fine. The enemies just waited and if the players was too close to him, he attacked.

 [SerializeField]
 private float rotationSpeed = 180;

 [SerializeField]
 private float movementSpeed = 1f;

 [SerializeField]
 private float meshRadius = 1f;

 private IEnumerator turnTowardsPlayerCoroutine;
 private IEnumerator moveTowardsPlayerCoroutine;

 private bool isDead = false;

 public float speed = 1;

 private int maxLife = 3;

 void OnTriggerEnter(Collider collider)
 {
     if (collider.gameObject.tag == "Player")
     {
         float playerDistance = Vector3.Distance(collider.transform.position, transform.position);

         if (playerDistance >= 2f * meshRadius)
         {
             turnTowardsPlayerCoroutine = TurnTowardsPlayer(collider.transform);
             moveTowardsPlayerCoroutine = MoveTowardsPlayer(collider.transform);
             StartCoroutine(turnTowardsPlayerCoroutine);
             StartCoroutine(moveTowardsPlayerCoroutine);
         }
     }
 }

 void OnTriggerExit(Collider collider)
 {
     if (collider.tag == "Player")
     {
         float playerDistance = Vector3.Distance(collider.transform.position, transform.position);

         if (playerDistance >= 2f * meshRadius)
         {
             StopCoroutine(turnTowardsPlayerCoroutine);
             StopCoroutine(moveTowardsPlayerCoroutine);
         }
     }
 }

 void OnDeath()
 {
     if (isDead)
     {
         return;
     }

     maxLife--;

     if (maxLife <= 0)
     {
         isDead = true;

         GameManager.Instance.NumKilledEnemies++;

         StopCoroutine(turnTowardsPlayerCoroutine);
         StopCoroutine(moveTowardsPlayerCoroutine);

         Destroy(gameObject);
     }
 }

 private IEnumerator TurnTowardsPlayer(Transform player)
 {
     while (true)
     {
         Quaternion targetRotation = Quaternion.LookRotation(player.position - transform.position, Vector3.up);
         targetRotation.x = 0f;
         targetRotation.z = 0f;

         transform.rotation = Quaternion.RotateTowards(transform.rotation, targetRotation, rotationSpeed * Time.deltaTime);
         yield return 0;
     }
 }

 private IEnumerator MoveTowardsPlayer(Transform player)
 {
     while (true)
     {
         Vector3 playerDirection = transform.position - player.position;
         playerDirection.y = 0;
         playerDirection = playerDirection.normalized;

         Vector3 deltaMovement = playerDirection * movementSpeed * Time.deltaTime;

         int layermask = LayerMask.GetMask("Environment");
         Vector3 movingTowards = transform.position - playerDirection * meshRadius + (new Vector3(0f, 0.1f, 0f));
         if (Physics.Raycast(movingTowards, Vector3.down, 0.25f, layermask))
         {
             transform.position -= deltaMovement;
         }

         yield return 0;
     }
 }

But after that, I wanted to add a predefined movement of enemies if nobody is close to him so I did something like this :

 private IEnumerator moveDefaultCoroutine;

 void Start()
 {
     moveDefaultCoroutine = MoveToPosition(new Vector3(transform.position.x, transform.position.y, transform.position.z + 3), 5);
     StartCoroutine(moveDefaultCoroutine);
 }

 private IEnumerator MoveToPosition(Vector3 newPosition, float time)
 {
     while (true)
     {
         float elapsedTime = 0;
         Vector3 startingPos = transform.position;

         while (elapsedTime < time)
         {
             transform.position = Vector3.Lerp(startingPos, newPosition, (elapsedTime / time));
             elapsedTime += Time.deltaTime;
             yield return null;
         }

         yield return new WaitForSeconds(2);
         elapsedTime = 0;

         while (elapsedTime < 0.8)
         {
             transform.eulerAngles = Vector3.Lerp(transform.eulerAngles, new Vector3(0, 180, 0), (elapsedTime / 0.8f));
             elapsedTime += Time.deltaTime;
             yield return null;
         }

         elapsedTime = 0;

         while (elapsedTime < time)
         {
             transform.position = Vector3.Lerp(newPosition, startingPos, (elapsedTime / time));
             elapsedTime += Time.deltaTime;
             yield return null;
         }

         yield return new WaitForSeconds(2);
         elapsedTime = 0;

         while (elapsedTime < 0.8)
         {
             transform.eulerAngles = Vector3.Lerp(transform.eulerAngles, new Vector3(0, 0, 0), (elapsedTime / 0.8f));
             elapsedTime += Time.deltaTime;
             yield return null;
         }
     }
 }

And here come the problem, when I'm adding this code the enemies disappear randomly... Did I did something wrong ?

Full code here : http://pastebin.com/tzGczv26

Thanks per advance !

PokeRwOw