The price of crude oil is the main factor in the price of diesel fuel, and fluctuations in the crude oil market greatly influence changes in diesel prices. Short-run factors that can cause divergences from this correlation include: supply shortages resulting from refinery outages, transportation issues, adverse weather conditions, and pipeline problems.
Longer-term factors that have been tied to variations in gasoline versus diesel prices include the different seasonal price variations for the two fuels, and differing tax rates.