The economic purists on this board would say it's simple "supply and demand" in action that drives the gas prices. But that's crap.
As FKent says, it's fear, speculation, and the current price of crude that drives the day-by-day, and even same-day increases at the pump as gas is on the rise; but its the price of crude and supply surpluses in crude that drive the cost back down, but MUCH, MUCH more cautiously (read: slowly...a penny every few days, a few cents over several weeks).
The gas companies have us over a barrel. We complain about the gas prices on the way up, begrudging the profits that are being made. The reality is that most of the profits are being made on the artificially slow decline, IMHO.
TJR