Why is my object tilting when moving?

Hello. I am using addRelativeForce to control a rectangle collider. However, it tilts whenever it is supposed to only move forward. How do I prevent this?

 var forwardMoveAmount = -30; //Sets forward  speed.
 var turnForce = 2; //Sets turning speed.
 var canDrive = 0;//Sets up ground detection (For now, works with all objects)
 
 function Start () 
 { 
     rigidbody.centerOfMass = Vector3 (0, 0, 0);//Keeps object upright. 
 }
 
 function OnCollisionEnter (col : Collision)
 {
     canDrive = 1;//Gives player control entering on ground.
     if (col.gameObject.name == "GoingUp")//Check if on a jump pad.
     {
         rigidbody.velocity = (Vector3.up * 30);//Makes player fly upwards. 
     }
 }
 function OnCollisionStay (col : Collision)//Gives player control on ground.
 {
     canDrive = 1;
 }
 
 
 function OnCollisionExit (col : Collision)//Removes player control off ground
 {
     canDrive = 0;
 }
 
 function Update () {
 
     var forwardMoveAmount = Input.GetAxis("Vertical") * forwardMoveAmount * canDrive;//Get input, multiplies it by movement amount and if the player can drive. 
     var turnForce = Input.GetAxis("Horizontal") * turnForce;//Get input and multiplies it by turnFroce
     transform.Rotate (0,turnForce * canDrive,0);//Rotates if canDrive is not 0
     rigidbody.AddRelativeForce (Vector3.right * forwardMoveAmount);//Applies relative force to object to move it
 }
 

Here is my whole player code in case it helps. Apologies if its a bit unprofessional, I'm quite new to unity.