New tech
When one drills in deeper water, the challenges are greater--the pressures are greater, the temperature of the oil is higher, and the stresses on the metals involved are greater.
The oil industry is creating ever-more technologically advanced equipment to deal with these issues, but the fact remains that it is virtually impossible to solve every new problem that may arise through computer simulations. If one tweaks one part of the equipment to make it stronger (to deal with the higher pressures, and greater temperature differential between the hot oil and the cold water), it can cause unforeseen problems with another system that interacts with it.
From The Gulf of Mexico Deepwater Horizon Oil Spill: Some Background and What It Means at The Oil Drum. A pretty decent review of the situation.
Accident? Not the word I would use either.
A Failure. And a costly one at that.