Frog starts moving without any keyboard input

using UnityEngine;
using System.Collections;

public class PlayerMovement : MonoBehaviour {

    private Animator playerAnimator;
    private float moveHorizontal;
    private float moveVertical;
    private Vector3 movement;
    private float turningSpeed = 20f;
    private Rigidbody playerRigidbody;

    // Use this for initialization
    void Start () {

        // Gather components from Player Game Oblect
        playerAnimator = GetComponent<Animator>();
        playerRigidbody = GetComponent<Rigidbody> ();
    }

    // Update is called once per frame
    void Update () {

        // Gather input from the keyboard
        moveHorizontal = Input.GetAxisRaw("Horizontal");
        moveVertical = Input.GetAxisRaw("Vertical");

        movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);
    }

    void FixedUpdate () {

        // If the player's movement vector does not equal zero...
        if (movement != Vector3.zero) {

            //...then create a target rotation based on the movement vector...
            Quaternion targetRotation = Quaternion.LookRotation(movement, Vector3.up);

            //...and craete another rotation that moves from current rotation to the target rotation...
            Quaternion newRotation = Quaternion.Lerp (playerRigidbody.rotation, targetRotation, turningSpeed * Time.deltaTime);


            //...and change the player's rotation to the new incremental rotation...
            playerRigidbody.MoveRotation(newRotation);

            // ...then play the jump animation.
            playerAnimator.SetFloat ("Speed", 3f);
        } else {
            // Otherwise, don't play the jump animation.
            playerAnimator.SetFloat ("Speed", 3f);
        }


    }
}