Maybe Mordekaiser or his Liches taught the Ruined King those fateful words that doomed the Isles.
So, maybe the catastrophe that doomed the Blessed Isles was a plot to resurrect Mordekaiser all along! It seems a bit convenient that the isles' artifacts amplified the king's words in such a way that the entire place imploded; maybe Mordekaiser's skull, or some clever setup, was the catalyst for the event.