The chart below represents the value of Brent crude oil since May, with the turning point at around the 28th of the same month. Prior to the gradual increase in value through most of May, there had been a sharp downward spiral fuelled by a massive selloff by major investors. These included the investment banks and hedge funds that had an agreement with OPEC to cut down production. In return, the investors would take long positions and prop up crude oil prices.
Apparently, OPEC had not honoured the deal, and the investors sold their positions causing the prices to drop. Starting in May, though, more investors opened long positions in anticipation of the May 28th OPEC meeting. According to the Commitment of Traders (COT) report, commercial investors hoping for further production cuts primarily drove long positions. When OPEC extended the same production cuts for a further 9 months, all major investors went short, causing the current downtrend.