Goldman Sachs, which runs the largest commodity index, the G.S.C.I., said in early August that it was reducing the index’s weighting in gasoline futures significantly. The announcement did not make big headlines, but it has reverberated through the markets in the weeks since and some other investors who had been betting that gasoline would rise followed suit on their weightings.

“They started unwinding their positions, and those other longs also rushed to the door at the same time,” said Lawrence J. Goldstein, president of the Petroleum Industry Research Foundation.

...

Goldman’s announcement on Aug. 9 was not the only downward pressure on prices that week, market participants stress. And while it may have played a part in sending prices down, the market would never have continued its downward trend unless supplies had loosened up, they say.

Also during that week, climatologists revised their hurricane forecasts, easing fears that oil supplies could be disrupted. And BP said it would still produce some oil from its field in Prudhoe Bay, Alaska, where leaks were being repaired. Meanwhile, the peak gasoline season was ending, and new supplies of ethanol were coming online.