Seriously? Are you asking me why there were explosions in a building full of propane tanks? Even in a building with zero propane tanks there are scores of thing that explode in fires.
I don't see any liquid steel.
I'm asking whether you think that gas tanks explode in such a way as to cause 7 similar impulses within 0.5s, and whether that accounts for the sequence of explosions that runs down the face of the building. What do you think is going on during the collapse? Is one exploding propane tank causing others to explode? If so, why does it occur when the fire has nearly died down, on different floors, and on lower floors not engulfed in flames? If you think they are dust puffs, the same questions, but also explain the point-like sources.
You'll see the molten metal on p14-15, under section 2.4