www.enreport.com

There are several explanations but let me summarize that
1. The realization of profits from commodity rush
2. Euro's fall, dollar's rise (read econbrowser link at the end)
3. Inventory levels
4. Demand Destruction
But to have a broader understanding, here is a bunch of explanations from FT (stated from some other source):

-”disappointing global economic headlines” (Platts), referring specifically to an unexpected fall in German industrial orders and the strongest rise in US unemployment since August 2010

-OPEC’s consideration to raise formal quota levels in June

-the death of Osama bin Laden and falling geopolitical risks amid “stabilising tensions” in the Middle East and the end of Nigerian elections

-the end of QE2 in June

-interest rate hikes in India and other Asian countries

-the rise of the dollar amid ECB indications of no imminent further interest rate hike

-the impact of higher fuel and commodity costs

-the general sell-off in commodities, with silver losing 25% of its value since April 25

-George Soros cutting his gold holdings

-a larger-than-expected build in US crude inventories

-a fall by 1.3 million b/d to 18.3 million b/d in US weekly implied demand figures

Ref:
http://blogs.ft.com/energy-source/2011/05/06/explanations-for-the-oil-crash/
http://www.econbrowser.com/archives/2011/05/lower_oil_price.html