The reason is simple. As the dwindling supply of oil in the ground becomes common knowledge, the price of oil will go higher. Of course not. The trend will stay strong throughout the time between now and then. They could be volatile because of the nature of the commodities market.
Oil prices can fluctuate wildly.
All sorts of things can influence the short-term price of oil. In a mega trend, a year or two can be considered short-term. Anything can happen short term, but it will not derail the mega-trend. In this case, down turns in price really are a buying opportunity. There are several problems with most alternatives though.