The Lost Radiants are commonly believed to have betrayed mankind on The Day of Recreance. Just why they did so remains unknown. They abandoned their Shardblades, which still seems incomprehensible to people of the modern era.
It has been speculated that the Day of Recreance was actually the day when Odium vanquished Honor, thereby splintering him. This potentially caused all Knights Radiant to lose the connections they'd had through their spren. Believing that they'd been abandoned, it's possible that the Radiants lost all hope and abandoned their duties as a result.