There are some real good discussions on a blog called The Oil Drum you might check out. It seams to be leaning:
1) The cement that sealed up the casing failed. This is common, so it has to be tested very carefully (more cement is pumped in if need be). Somehow BP though the cement job was sound when it wasn't. After that as the removed some mud in preparation for closing the well this released pressure and the oil/gas rose up and blew out.
2) Human error. Once the cement job was known good they got sloppy and stopped watching carefully. There would have been clear indication of the impending blowout because extra mud had to come out first. They missed this. So nobody pushed the button.