Due to rapid technological advances in shale oil extraction (ie fracking), North American companies have radically changed the daily global supply of oil, which of course puts downward price pressure on it.
Much of the recent price drop can be defined in terms of Saudi's getting into a price war with these NA companies, trying to push them off the market.
Not only that, but this technology is a long-term game changer, ie it will be only a matter of time before all the world oil players (Russia, Venezuela, etc) co-opt this tech and start extracting much cheaper oil from their fields, which of course the markets are anticipating as well.
I have to admit I'm surprised. I totally bought the idea that oil would never be this cheap again for fundamental physical reasons. But perhaps that made it a bubble. A true bubble must have a "story" about how it's "different this time" that manages to convince huge numbers of people.
I recall years ago though reading a debate between a peak oil "doomer" and someone saying climate change was the bigger problem. The latter kept saying we have more oil at higher prices and that the real problem is we have "enough carbon to destroy the world." If we really got desperate and decided we didn't want Florida we could do deep in situ conversion of massive conventionally unreachable reserves of very deep coal into gas ("water gas reaction"), then liquids. That could, he said, keep our liquid based transport economy going for thousands of years but would send Earth back to the Permian.