It's hard to tell what actually went wrong. Was it really his bike handling skills or did he fear falling during those downhills sections? Many times when they zoomed in on him there was a gap of 5-10m with the cyclist in front of him so he must have spent a lot of energy. But was this sufficient to have that collapse at the end? Maybe he also experienced a dip at the same time? It was his first rest day ever so he doesn't know how his body react to this, certainly not after such a long absence from pro cycling. Also, that frustration didn't help him either although you could see why he was angry. Almeida was not helping him when he dropped and he lost him a second time as well. Apart from those 2 dips, he did put up a good fight. During the last 5km he basically kept the same pace as Bernal and Buchmann while the others lost 30 seconds.