Two things come to mind: If the binding is already sewn on, does it need to come off if you are only going to quilt the borders? It does occur to me that the quilt might not be square and that might cause the wonkiness, but it also may be that, as others have suggested, the uneven quilting is causing the strange shape. Also, the advice to dampen the hanging and pin it securely to the carpet and let it dry might just stretch it into shape.