At 30,000psi of pressure you don’t need an explosion. Crap just starts coming apart.
The gas ignited when it got to the surface and settled into the rig.
From all acounts here is the screw up.
They assumed the cement held and they opened the BOP with that assumption. The cement didn’t hold and a gas bubble shot up to the well.
Sounds reasonable. Your input will be helpful in a new thread. Ping coming ....