It cracks me up to see how some people just don't get it.
A) The riders didn't "choose" not to race, but were told by their teams that the descent had been neutralized, to put on something warm and proceed together with caution. Only some chose to slip away. I don't believe Nairo and DS were unaware of what the others were doing (though this is just my opinion), but I'm certain that the others were ignorant of what Nairo and company were up to until it was too late. At any rate Nairo gained an advantage he was able to capitalize further on, only because his rivals weren't racing at a critical moment. For the Colombian's (or any athlete's) own pride this was not fortuitous, since any victory should be morally accompanied by direct and even confrontation throughout.
B) The lack of collaboration behind was due to the economy of the course and anarchy that the organizion had thus created at that point.
C) if the race had not been interefered with by the organization, there is no way that Nairo gains the time he did on his direct rivals. Had they all started together at the base of the final ascent, as would have almost certainly been the case, tomorrow's stage would have seen Quintana, Uran and Aru all within seconds, instead of the race for second place we actually have.
Thus Quintana's victory in this Giro will always be left with a "what if" in regards to stage 16, which is a shame both for himself and the Giro.