It was actually an oil leak. From what I understand there was problems on the oil rig and guys on the rig tried to contain it but failed. When they realized how bad it was going to be they called in to thier bosses and said it must be shut down. They rufused and the rig exploded. The explosion broke the pipeline loose. Being under pressure oil spilled into the ocean directly from the well. What angers me is 7 people died, and that was never really talked about. It was a horrible situation, but it is being taken care of.
A number of things, one main thing is a failure in the emergency shut off valve which is supposed to trip and close incase of any emergency to stop the flow of oil. This didn't work allowing millions of gallons of oil to flow into the ocean. The other aspects of it are still unknown to my knowledge.
I'm not sure if anyone knows exactly what caused it yet.
Greed. BP didn't follow proper safety regulations.