http://www.rita.dot.gov/bts/airfares/programs/economics_and_...
To add to this point. My wife just got a ticket from Atlanta to Indianapolis for 49 dollars round trip. . . You can't drive from Atlanta to Indy for 49 dollars.
Norwegian has incredible fares also, many I've seen for less than $400 round trip Los Angeles to various destinations in Scandinavia.
It's simply demand in the holiday case, but it's still ridiculous. At other times, high demand does not necessary apply and they do all kinds of things to generate demand and compete for butts in seats, especially when fuel prices are low.
But with a high-demand holiday, at a hub almost entirely monopolized by one airline, there is no ceiling to how high they can raise price in response to that demand.