Twice as strong? I don't think so. The stronger dollar may have contributed a little, but the rest of the difference has to be due to speculation.
If you go back over the last few years you'll see almost identical spikes and declines in the price of oil from May to September.
I drove halfway across the country in the aftermath of Hurrican Katrina a few years ago in September, and it was amazing to see the price of gasoline drop almost by the hour as more and more refineries came back on line after the storm.