From the American Association for the Advancement of Science via their ScienceNow site:

"The $350 million Deepwater Horizon was not operating near its design limits on 20 April when uncontrolled oil and gas shot up its drill pipe and ignited, eventually sinking the vessel. It was designed to drill through as many as 9144 meters of rock in as much as 2438 meters of water. Last year, it had drilled the world's deepest well at 10,680 meters of rock and water. But at the time of the blowout, the well extended only 5486 meters beneath the sea floor in 1500 meters of water."

http://news.sciencemag.org/scienceinsider/...

The consequences of this incident are horrific with 11 deaths plus the developing evironmental impact, but the causes are the same causes as many other industrial accidents. I would label that cause, generally speaking, as the myth of "industry self-regulation".