Rivka has a fantastic post connecting the horrific rumors about New Orleans that cropped up post-Katrina with social psychology:

Cognitive dissonance gets particularly ugly when reality collides with the just world hypothesis, the belief that "the world is an orderly, predictable, and just place, where people get what they deserve." Faced with tragedy, victimization, or injustice, just world believers have four options to reduce the cognitive dissonance: they can act quickly to help relieve the victim's suffering (restoring the justice of the situation), minimize the harm done (making the tragedy a less severe blow to their beliefs), justify the suffering as somehow deserved (redefining the situation as just), or focus on a larger, more encompassing just outcome of the "poor people will receive their rewards in heaven" variety. The first response - the only actually helpful one - isn't always possible. Unfortunately, the latter three pretty much always are.

The aftermath of Hurricane Katrina confronted Americans with a constant parade of images of suffering. Terrible suffering, to extremes hardly imaginable in a wealthy and highly developed society. American citizens dying of thirst, dead bodies lying uncollected on the streets of a major city, elderly people and children penned into the Convention Center and the Superdome in unimaginable squalor, denied even the most basic of aid from their government. There was no immediate way for private citizens to help them. Faced with those horrific images, most of us had powerful reactions of grief, rage, shame, fear, pity. In others, however, the images of Katrina caused cognitive dissonance too great for them to tolerate. Where is the "just world," when wheelchair-bound grandmothers die of thirst? How to maintain, watching the abandonment of New Orleans victims to day after day of imprisonment without relief, the conviction that this is the "greatest country in the world"?

So rumors about the depraved criminal nature of stranded New Orleans citizens spread like wildfire...

That's good stuff, though I doubt "cognitive dissonance" was all that was going on after Katrina hit, when (parts of) the country became temporarily obsessed with digging up horror stories. Bill O'Reilly's various remarks—about blacks who "planned" to stay around so that they could get some quality looting-time—deserve, I think, a less charitable interpretation. But still, combine that with Mark Schmitt's theory that, in times of crisis, people genuinely need to believe that there's a competent leader in charge of the situation, even if there's no such thing, and you really have something. A wildly over-generalized and ultimately quite frightening something, but there you go.