That is the question that the political chattering classes are asking, following one of the biggest examples of polling failure since Dewey beat Truman in 1948. Was it her tears? Was it Barack Obama's rather peevish line about her being likeable during the last debate? Was it the presence of Bill Clinton on the ground in New Hampshire? According to Joshua Holland of AlterNet, the answer to all of those is "No." He attributes it to far more mundane factors: turn-out, organization, and Biden's supporters going to Clinton.
Now, the media has a vested interest in pushing the line that it was her supposed "tearing up" during a question and answer session in New Hampshire that made the difference. This is because it makes them look important and because it is an easy story to explain. It has good visuals, it has a narrative, and it encourages debate among viewers and readers. (Were her supposed tears genuine? Was this a planned stunt? Is she human after all?)
The points that Holland is making are much less compelling from a story-line point of view. Turn-out, organization, and Biden's supporters going to Clinton do not make for good television and do make for a pretty boring story. In the final analysis, though, campaigns are won far more often because of turn-out and organization than are won because of a "television moment", George Allen's defeat in Virginia notwithstanding.
So when you hear political pundits confidently telling you why Hillary Clinton won New Hampshire, just remember this: If they were so wrong about what was going to happen in New Hampshire, why should you believe them when they tell you what happened in New Hampshire?