Changing child's rotation changes parent's position

Tried to search the answer for it but found nothing so i think i am missing something very obvious here.

I have a main camera as a child of a vehicle. In the script attached to the camera i am reseting camera's rotation so it is not rotating with the vehicle, just follow it:

 transform.rotation = Quaternion.Euler(0,-180,-180);

However the moment the vehicle changes it's rotation, all hell breaks loose: this single line of code somehow magically causes the vehicle to fly out of the screen (to be exact: it changes it's position by hundreds of units per second.)

Can someone explain to me what is happening exactly ? Very thanks.

EDIT: The vehicle is a space ship (no wheels, no physics) actually and it moves forward with:

 transform.Translate(Vector3.forward * speed * Time.deltaTime)

and to the sides:

 mousePosition = Input.mousePosition;
 mousePosition.z = 50; //distance to camera
 touchPosition = Camera.main.ScreenToWorldPoint(mousePosition);
 direction.x = touchPosition.x;
 direction.z = transform.position.z; //player controls movement
                                     //on x-axis only                
 transform.position = Vector3.Lerp(transform.position, direction, 0.1f);