Camera randomly turns left or right

I have the main camera as a child of my player, where the following code has it so that the player can look around. For some reason, I have this issue where if I walk into some stairs or if I walk into a door, the camera rotates by itself (left or right). I might just be stupid, but can someone help?

using UnityEngine; using System.Collections;

public class FirstPersonController : MonoBehaviour {

 public float playerSpeed = 5.0f;
 public float mouseSensitivity = 5.0f;
 public float verticalRange = 60.0f;

 float verticalRotation = 0f;

 // Use this for initialization
 void Start()
 {
     Cursor.visible = false;
 }

 // Update is called once per frame
 void Update()
 {
     //Movement
     float xAxis = Input.GetAxis("Horizontal") * playerSpeed;
     float yAxis = Input.GetAxis("Vertical") * playerSpeed;

     if (Input.GetKey(KeyCode.W) || Input.GetKey(KeyCode.A) || Input.GetKey(KeyCode.S) || Input.GetKey(KeyCode.D))
     { 
         transform.Translate(xAxis * Time.deltaTime, 0, yAxis * Time.deltaTime);
         GetComponent<AudioSource>().UnPause();
     }
     else
     {
         GetComponent<AudioSource>().Pause();
     }
     

     //Camera
     float rotateHorizontal = Input.GetAxis ("Mouse X") * mouseSensitivity;
     transform.Rotate (0, rotateHorizontal, 0);

     verticalRotation -= Input.GetAxis ("Mouse Y") * mouseSensitivity;
     verticalRotation = Mathf.Clamp (verticalRotation, -verticalRange, verticalRange);

     Camera.main.transform.localRotation = Quaternion.Euler (verticalRotation, 0, 0);
 }

}