In short demand has gone down because of more efficient vehicle and not so rosy economic picture worldwide.
Now coming to second part, There are a number of conspiracy theories floating around. Even some oil executives are quietly noting that the Saudis want to hurt Russia and Iran, and so does the United States — motivation enough for the two oil-producing nations to force down prices. Dropping oil prices in the 1980s did help bring down the Soviet Union, after all. But there is no evidence to support the conspiracy theories, and Saudi Arabia and the United States rarely coordinate smoothly.