GameObject rolls towards Waypoint

Hi, Why does my capsule game object (or box) roll towards a waypoint, it has a ridged body and capsule collider attached with the below script ?

Thanks.

 #pragma strict
 
 var waypoint : Transform;
 var speed : float = 20;
 
 
 function Start () {
 
 }
 
 function Update () {
 
 var target : Vector3 = waypoint.position;
 var moveDirection : Vector3 = target - transform.position;
 var velocity = rigidbody.velocity;
 
 if(moveDirection.magnitude < 1){
  velocity = Vector3.zero;
 }else{
  velocity = moveDirection.normalized * speed;
 }
  rigidbody.velocity = velocity;
 }