My GUESS, because I surely don’t KNOW, is that they fractured the formation while testing the cement job and the well drank enough of the weight material so quickly that the well blew out before they realized what was happening. BOP won’t close because of some hardened tool being in exactly the wrong place at the wrong time.
That was my thought. One of the devices just happened to be at the shear point when the gas bubble burped. Sometimes $hit just happens.