Ok, I have a theory.
They were loading RP-1, Supercooled LOX, and GHe.
LOX tank was chilled but the Kero tank was not. Aluminum gets stronger at cryo
temps.
Partway through loading a GHe tank lets go. The flight kaboom some time back
had a full LOX tank so little space for the GHe to go, blew the LOX tank. This
time, half-full LOX tank is enough expansion space (just) to avoid blowing the
tank immediately. But the warmer tank bottom or feed line down weren't as well
chilled and are therefore weaker, and rupture. *ping*. And pressure drops in
LOX tank.
So the LOX and GHE drop into the RP-1 tank. They don't have time to mix much
to detonable mixture. Mixture of ongoing GHE venting and LOX boiling from the
RP-1 raises combines tanks pressure again and the warmest barrel sections, the
RP-1 tank outer walls, rupture. Possibly, a small volume reached explosive
mixed conditions and turbulence or static or impact detonates a small explosion
in a bigger mass of still mixing materials.
Some of what's ejected out is LOX - white fluffy non burnie cloud that
evaporates in the heat. Some is RP-1 and just burns. Some is mixing RP-1/LOX
and burns REALLY WELL.
When the RP-1 hoop section lets go you have say 100 PSI times pi/4 times 144
squared or say 1.6 million pounds downwards force on the lower RP-1 tank dome
and the Vacuum Merlin, held up by the interstage. The Interstage structure
fails due to gross overload, Merlin and tank dome are forced down into Stage 1
LOX tank and destroying the first stage.
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