GameObject instances affecting each other

I have a function in a script attached to the player object which fires a cannonball when the player hits space:

 void Fire()
 {
     Object cannonBall = Resources.Load("cannonball");
     GameObject shotInstance = (GameObject)Instantiate(cannonBall);
     shotInstance.GetComponent<Rigidbody2D>().AddForce(new Vector2(Mathf.Cos(angle / 90f) * cannonForce, Mathf.Sin(angle / 90f) * cannonForce));
 }

This works fine by itself, I can fire many cannonballs in rapid succession and they do not interact.

However, when I use nearly the same function in a different script attached to an enemy, it interacts with the player's shots.

     void Fire()
     {
         var angle = 20.0f;
         Object cannonBall = Resources.Load("cannonball");
         GameObject shotInstance = (GameObject)Instantiate(cannonBall);
         shotInstance.GetComponent<Rigidbody2D>().AddForce(new Vector2(-1 * Mathf.Cos(angle / 90f) * cannonForce, Mathf.Sin(angle / 90f) * cannonForce));
     }

If the enemy's Fire() function is called after the player's Fire() function, it adds the force to both cannonballs. I find this very confusing as the player's cannonballs do not interact with each other. None of the objects are static. Can anyone tell me what I need to do here so that the enemy shots do not affect the player's?

Edit: the player cannon exists as an object on the scene, the enemy's is instantiated in the same way as the cannonball by an enemy loader script