With the rumoured info available, here's my take: a gas leak caused an initial, smaller explosion and subsequent fire, that initial explosion's concussive blast disturbed what had previously been settled cocoa dust across rafters/pipes/vent shafts, and when this cloud of dust was sparked, the entire building became a pressure bomb in effect.
GoliathTCB t1_jdv3v4e wrote
Reply to All 7 Pennsylvania chocolate factory explosion victims found by Substantial-Pass-992
With the rumoured info available, here's my take: a gas leak caused an initial, smaller explosion and subsequent fire, that initial explosion's concussive blast disturbed what had previously been settled cocoa dust across rafters/pipes/vent shafts, and when this cloud of dust was sparked, the entire building became a pressure bomb in effect.