The September 24 SF Chronicle has an article entitled Oil's slippery slope:  Moves by monied speculators bear heavily on prices that explains why energy prices might be dropping:

[...]
Deep-pocketed speculators helped fuel the steep rise in oil and natural gas prices as they pumped money into the markets for those commodities. Now they're pulling some of their money out, causing prices to plummet.
[...]
But a crucial role in the market is played by big investors who never have the slightest intention of taking tanker cars full of oil or gas. These are high rollers, such as investment banks and hedge funds, that buy contracts for future delivery of commodities as bets on the direction prices are moving.

The movement of all that speculative cash isn't the only reason energy commodities are tumbling. But analysts say the behavior of big market speculators has amplified and speeded up the drop.
[...]

The Nieman Watchdog website (a project of the Nieman Foundation for Journalism at Harvard) has had some good coverage of oil prices lately:  on politics and on speculators.