Geological / environmental event
The impact from the collapse of Ygníheim caused such a massive shift in the realm, that the underside of the debris upon which Vyngaard existed, cracked and created the Feyfjords.
The massive shift in the very piece of debris that the world of Vyngaard existed on, caused the underside of it to crack, and it was split into equal shape as Vyngaard above. The essence of the wood of the debris from the World Ship Aldinnhalldr seeped into the world, and magic was made to roam with chaos.