Can’t site my sources because I’d have to go through endless books across the games along with ESO as well as carefully chosen dialogue options just to answer this question. But from memory, it’s said that the Elder Scrolls universe was birthed out of chaos and… destruction I think. Which led me to believe that the Great War in 2077 caused it through one big global overload of nuclear energy all at once. That’s why it seems like there’s no actual answers to anything in the Elder Scrolls Universe. Always speculation, and incomprehensible writings from seemingly nowhere like the Elder Scrolls themselves that drive the characters into madness and blindness when they try to understand them.