If you were a moderately right-of-center voter who was fed up with Cuccinelli and told pollsters you wouldn’t vote for him, it is possible that when you walked into the voting booth, partisanship took over. Maybe these voters just skipped the race, or maybe they voted Cuccinelli. Either way, the result would be the same: McAuliffe’s share drops from what the polls projected.
This would also explain why there was no accompanying drop-off for Democrats down-ballot. Ralph Northam was a pretty bland, faceless candidate for lieutenant governor; he was a generic Democrat and so turned off fewer of the center-right voters who were turned off by Bishop E.W. Jackson. Obenshain didn’t see any drop-off because these voters were probably voting specifically for him (who was something of “generic Republican” anyway).
A related explanation would parallel the “Shy Tory” factor, a reference to a phenomenon in British polling in the 1990s, when the Tories would routinely poll worse than their actual electoral showings. After some investigation, it was thought that voters were simply embarrassed to tell pollsters that they were voting for the Tories.
Was there a “Shy Cuccinelli” factor? Again, it’s impossible to know, but the explanation is at least consistent with what we saw both at the top of the ticket and down-ballot (people were not shy about supporting Obenshain, and really did oppose Jackson).