It's caused by inflation, folks. The government (the Federal Reserve actually) has been inflating the currency since 2001. That means the currency is worth less, stuff costs more, with commodities going up first, especially imported commodities. That means oil.
And the inflation, brought to you by the low interest rates that pumped up real estate, is heating up the economy and increasing our own demand for oil.
China's currency is tied to the US dollar, so, surprise, surprise, so is Chinese demand for oil.
Its bad public policy again.