why does character controller accelerate off ledges?

Hey, so I'm using a character controller for character, but the typical movement found in the docs doesn't allow him to change direction in air.

     /// This script moves the character controller forward 
 /// and sideways based on the arrow keys.
 /// It also jumps when pressing space.
 /// Make sure to attach a character controller to the same game object.
 /// It is recommended that you make only one call to Move or SimpleMove per frame.    
 
 var speed : float = 6.0;
 var jumpSpeed : float = 8.0;
 var gravity : float = 20.0;
 
 private var moveDirection : Vector3 = Vector3.zero;
 
 function Update() {
     var controller : CharacterController = GetComponent(CharacterController);
     if (controller.isGrounded) {
         // We are grounded, so recalculate
         // move direction directly from axes
         moveDirection = Vector3(Input.GetAxis("Horizontal"), 0,
                                 Input.GetAxis("Vertical"));
         moveDirection = transform.TransformDirection(moveDirection);
         moveDirection *= speed;
         
         if (Input.GetButton ("Jump")) {
             moveDirection.y = jumpSpeed;
         }
     }
 
     // Apply gravity
     moveDirection.y -= gravity * Time.deltaTime;
     
     // Move the controller
     controller.Move(moveDirection * Time.deltaTime);
 }

Here is the code that allows me to change direction in air (credit:ANAMOLOUSUNDERDOG)

 #pragma strict
     public var jumpSpeed:float = 8.0;
     public var wallJumpSpeed:float = 0;
     public var gravity:float = 20.0;
     public var speed:float = 6.0;
     public var jumping:boolean = false;
     private var moveDirection:Vector3 = Vector3.zero;
     private var controller:CharacterController;
 
     function Start()
     {
             controller = GetComponent(CharacterController);
     }
 
      function Update()
     {
     
     
             moveDirection = new Vector3(Input.GetAxis("Horizontal"), moveDirection.y, Input.GetAxis("Vertical")); // make sure the old value of moveDirection.y won't get lost
             moveDirection = transform.TransformDirection(moveDirection);
          
             // don't apply speed to moveDirection.y. apply it only to x and z
             moveDirection.x *= speed;
             moveDirection.z *= speed;
            
             if (controller.isGrounded && Input.GetButton("Jump"))
             {
                     moveDirection.y = jumpSpeed;
             }
 
         controller.Move(moveDirection * Time.deltaTime);

}

I'm assuming it has something to do with moveDirection.y but what happens is, when walking straight off a ledge, the player accelerates downwards quickly. This does not happen when jumping, only going off ledges. What could it be?