Remember that OPEC countries account for the global 1/3 oil production.
Supply & Demand
There has been a boom in oil drilling in the US (North Dakota & West Texas). Also the booming US Shale drilling has been another factor to reduce the demand of oil. With US becoming ever more less dependent on Gulf oil, the overall global demand of the gulf oil has reduced to certain extent and the threat from US booming oil drilling has further added to the apprehensions of Gulf losing more demand in the coming future.
The domestic production in US has reached almost 8.7 million barrels per day and its imports from OPEC countries have reduced by over half since 2008. This is coupled with increased oil production from Saudi Arabia and also Iraq and Libya.
Further, demand in Europe has decreased with persistent industrial slowdown & once Japan restarts its nuclear power plants, the demand for oil will further decrease.
Why the prices fall remained unchecked ? What escalated or continued the process of fall ?
The OPEC members are not in agreement over the steps to be taken. This is because the threat from US booming oil sector is large which can run at break-even price of upto $ 60 a barrel. Several members of OPEC - Saudi Arabia, Iraq, Iran, Kuwait have thus lowered the prices in order to retain their market share in Europe and Asian markets.