That's for sure not stage separation, that's an explosion from the FTS rupturing the ship tanks.
If it was the FTS wouldn't the flight control systems send a message back to the ground saying "things are going sideways here, FTS Activated"
Eh I'm thinking more it was a reentry explosion from pressurized tanks. Engines had failed a while before then.
If it was the FTS wouldn't the flight control systems send a message back to the ground saying "things are going sideways here, FTS Activated"