It says China didn't grow as much as expected, etc. Well if that'd be a simple offer/demand, the price would have not collapsed so rapidly. Oil is not like housing where bubbles comes and go. It's usually pretty steady unless something happens (9/11, 1982, etc).
Also, price seems to be recovering now which indicate, to me, that it's not an offer/demand issue. The oil collapsing and recovering in a ~6 months period is just plain weird.
If I had to bet, I'd say it's due to ISIS and their access to oil and selling it at a quarter of the ongoing prices. I read a story about that a few months back how they had access to major oil field and were selling it insanely cheap on the black market to finance their operations.
Those are all wild ideas. But things are just weird, I'm telling you.