Problem with Rigidbody2D.angularVelocity

Hi, I have a sprite that the player controls. I move the sprite (with Rigidbody2D attached to its game object) using the rigidBody2D.velocity = someVector;

For reasons unknown to me at times the rigid body appears to receive a large amount of torque from somewhere. It starts spinning slowly coming to a stop. I do not change any variable other than the velocity nor do i call any function on the rigidBody. The physics system must be doing it. My question is why? Here is the code that moves a rigidbody.

 public class CharacterMovement : MonoBehaviour {
 
     private Rigidbody2D characterRigidBody;
     public float speed;
     public Vector2 inputDirection;
 
     void Awake () 
     {
         characterRigidBody = (Rigidbody2D)GetComponent (typeof(Rigidbody2D));
     }
     
     // Update is called once per frame
     void FixedUpdate () 
     {
         characterRigidBody.velocity = inputDirection * speed;
     }
 }

Here is the code that used the CharacterMovement class.

 private CharacterMovement motor;
 
 void Awake () 
     {
         motor = (CharacterMovement)GetComponent (typeof(CharacterMovement));
     }
 
     void Update () 
     {
         Vector2 moveDirection = new Vector2 (Input.GetAxis ("Horizontal"), Input.GetAxis ("Vertical"));
         float speed = moveDirection.magnitude;
         moveDirection.Normalize ();
 
         if (moveDirection != Vector2.zero)
             transform.rotation = Quaternion.FromToRotation(Vector3.right, new Vector3(moveDirection.x, moveDirection.y, 0));
 
         motor.inputDirection = moveDirection;
     }

I can't work out where this mysterious force comes from. Brushing up against another rigidbody such as an NPC reduces the force and brings the player to a standstill.