While it's still unclear exactly what happened, Miller’s older brother Dylan said Balin was lead rope soloing — a way to climb alone while still protected by a rope — on a 2,400-foot (730-meter) route named Sea of Dreams. He had already finished the climb and was hauling up his last bit of gear when he likely rappelled off the end of his rope, Dylan added.
Making knots into the ends of your rappel rope is such a basic basic basic thing that gets drilled into climbing rookies from day one. And yet it still happens even to highly experienced climbers who have done this a million times.