Best rumor as to sequence of events, heard from a good source, but still unconfirmed.
Production liner was cemented.
Two cement plugs were placed in the wellbore.
Riser was displaced to seawater
All hell broke loose
After the cement plugs were placed, the well should have been dead. So either the plugs failed, or the liner collapsed, which then caused the blowout. I assume the crew noticed the well was alive, tried to close the BOP, it failed, gas and oil came to surface, and you know the rest.
There are two phases to this disaster, first, why was the well alive when it was thought to be dead, and, why did the BOP fail to close?
I can assure you, some serious changes will come about from this disaster.