I'd like to hear something definitive about the locations of the LOX and RP-1 pipes in the strongback. Does anyone have any official information?Haven't viewed the video? It's only 9 minutes. The authors of the video suggest the fuel source as leaked kerosene or pipe insulation, seals, gaskets, or even the metal itself, potentially saturated or caked in recondensed LOX. In a previous video it is discussed how strongback kerosene pipes may at times flow in the reverse direction.
The umbilicals going across to the rocket are certainly lower down than the ignition point so there doesn't appear to be an obvious point in having pipes higher than that. I would not have expected leaking kerosene (which is of course still liquid at ambient temperature) to mix enough with any form of oxygen to explode initially with that much speed and energy. One can see how it burns later; the part which has been hit with LOX burns fiercely, but the part which is mixing with air looks more like a rain of fire. If it were leaking as a spray from a small aperture under pressure, it's possible that it could mix well with the oxygen, but as RP-1 loading was supposed to be complete and the wind was away from the strongback it would seem strange to me that fuel under pressure was still in the vicinity.