Not one to dwell on but I'll throw my dusty old training cap into the ring after the release of the NTSB prelim, which shows the left engine separating upon rotation. The term is pilot-speak for take-off, as the aircraft is rotating about its horizontal axis.
Boeing checklists ~ this is a Douglas aircraft re-named ~ always included one that read ENGINE FIRE, SEVERE DAMAGE OR SEPARATION and unfortunately this crew had all three going on at the same time. Surprisingly, podded jet engines actually are designed to fall off in extremis, in order to avoid them further damaging the wing they hang from... or indeed setting fire to it.
The engines are suspended from a cantilevered pylon that is pitched forward to aid in stability if nothing else: as per a dart. In turn though that pylon is attached to the wing at two points that are generally the forward and aft spars on which it is based.
Ideally in the circs you'd want the engine to separate from the pylon, but here due a corrosion issue the entire assembly has ruptured and set a fire burning that is fuelled by the, well, fuel stored in the wing.
The engine is producing the same thrust throughout the take-off, which begs the question as to why it did not fall off sooner. The act of rotation tho' not only pitches the mass of the engine up, but simultaneously applies force in another direction due to its (considerable) gyroscope inertia.
Old people and airframes, twisting whilst lifting never ideal.
