Wasn't it the destruction of Ashyn, caused by the Surges that caused the Radiants to abandon their Oaths. The truth about the original Voidbringers was emotionally devastating, of course, but the logic of it was the destructive capacity? Plus, Honor was going mad and raving at them, the Radiants' worthiness was being questioned, there were Odium's forces active despite the Heralds' reassurance that the war was over.
It was probably the straw that broke the camel's back