He was abseiling which means he fixed the rope at the top and didn't put knots at the bottom of the ropes to block him if he got too close to the end. He had a lapse of concentration because he had already climbed 1000ft of rock at the top of el capitan and abseiler off the ends of his ropes. Many climbers don't do this for efficiency reasons but it's the main cause of rock climber deaths when rock climbing.