From Mike Williams statement-

When first drilling down, a BP manager ordered a faster pace Due to the faster pace, the bottom of the well split open and the well had to be abandoned, costing BP millions.
Pressure to get the job done was increased.
A rig accident damaged the annular at the top of the blowout preventer. The crew sealed the pipe, but too much force was applied, and chunks of the rubber annular came up in the drilling fluid.
The supervisor said it was no big deal.

"There was apparently no way to know how much damage was done. When there was a meeting to discuss how they were going to seal the well. Williams said a manager from BP changed the process at the last minute and 'communication broke down'."

So, in typical rash English fashion, "into the valley of death rode the 600."