Why does an object start rotating on frozen axes ?

I have a game object like this.

![alt text][2]

At the start of the game, it starts to rotate using this code

 protected void FixedUpdate()
 {
     _rootRigidbody.angularVelocity = Vector3.right * 100f;
 }

After I add 2 child objects with the grid collider it starts rotating on the frozen axes. Depending on a different mesh in a different direction. Why is this happening ?