There is a really great paper to be written going back and reinterviewing these 3000 people with an eye toward figuring out just why this happened. Just saying:
USC Dornsife/LA Times: The Presidential Election "Daybreak" Poll: "The 2016 USC Dornsife / LA Times Presidential Election Poll represents a pioneering approach to tracking changes in Americans' opinions throughout a campaign for the White House...
...Around 3000 respondents in our representative panel are asked questions on a regular basis on what they care about most in the election, and on their attitudes toward their preferred candidates. The "Daybreak poll" is updated just after midnight every day of the week.
This chart tracks our best estimate, over time, of how America plans to vote in November. The final blue and red figures on the right side of the chart represent our most recent estimates of Hillary Clinton's vote (blue squares) and Donald Trump's (red diamonds). These estimates represent weighted averages of all responses in the prior week. The gray band is a "95-percent confidence interval". Figures lying outside the gray band mean that we are at least 95% confident that the candidate with the highest percentage will win the popular vote.