Why does this script make my camera jump when I move back?

Alright, so I got a script from somewhere that I tweaked a bit, and it makes my character jump when I hit S, or Down while looking at the ground. Can someone please help me fix this bug?

 using UnityEngine;
 using System.Collections;
 
 public class fps_move_player : MonoBehaviour {
 
     public float speed = 50.0F;
     public float jumpSpeed = 60.0F; 
     public float gravity = 60.0F;
     private Vector3 moveDirection = Vector3.zero;
     
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
          CharacterController controller = GetComponent<CharacterController>();
         // is the controller on the ground?
         if (controller.isGrounded) {
             //Feed moveDirection with input.
             moveDirection = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
             moveDirection = transform.TransformDirection(moveDirection);
             //Multiply it by speed.
             moveDirection *= speed;
             //Jumping
             if (Input.GetButton("Jump"))
                 moveDirection.y = jumpSpeed;
  
         }
         //Applying gravity to the controller
         moveDirection.y -= gravity * Time.deltaTime;
         //Making the character move
         controller.Move(moveDirection * Time.deltaTime);
     }