Emily Thorson, who has organized in Iowa, has suggested that it was a fluky weirdness about the caucus system:
The TV coverage I've been watching has implied that New Hampshire is a crazy comeback surprise and Iowa is somehow the "real" result. I think they're wrong. Iowa is the anomaly, because of the bizarre public forum that is the Iowa caucus. You know why Hillary does worse in a caucus? Because women who are leaning Hillary go to the caucus with their husbands, and he says "Let's go for Obama" or "Let's go for Edwards" and she says "Well, all right then" because she doesn't want to spend the next hour sitting alone in the Hillary group. I've sat through a caucus. This is how it works.
I'm also hearing that the Clinton organization in Iowa was a bit of a disaster: no hispanic literature at all, basically no student outreach, and a host of other, smaller problems. That doesn't negate what Emily says, but it might have amplified the effect.