In the past year not only prices of oil have fallen, but also for other commodities. For example, iron ore & rubber prices decreased more than 30%, Coal, Soybeans, Rapeseed & Palm oil about 20-25%. Common reasons that explain the fall of commodity prices include the rise of the USD (due to rising interest rate expectations), the economic slowdown in China, cost cuts induced by a fall in fuel price. But some foods, like beef, chicken meat and coffee showed significant uptrend. Also, some grain prices were rising, especially by the end of the year. Apparently, commodity markets are influenced by their own factors (such as harvest, shipping and warehousing situation, local export bans or changes in taxation etc.) and not only macroeconomic ones.
(Barcharts shows 1 year percent changes, for latest (Dec-14) price data see the table. Select desired commodity in the table to see long-term and short-term trends)
Source: IMF World Economic Outlook