Speculation (worth what you paid for it
):
Without knowing more about the building of the pack itself, my guess based on what I've seen of various 18650 builds here and the Vpower pack I once had, is that there was insufficient insulation between the inter-cell connections for parallel cells and the cell can edges underneath them, and something (heat, vibration, pressure, etc) caused what insulation there was to be breached, causing a short across a cell (and thus the entire parallel group). That would then heat all those cells.
What happens after that depends on where the cells are
--if in the middle of the pack they'd heat up more cells around them, and the heat has nowhere else to go but into other cells
and how the charger and BMS reacts to the sudden voltage drop
--if the charger continues to pour current at a high rate into the pack, and the BMS doesnt' shut off charging when it detects the now-shorted cell group,
then as the other cells aroudn that group heat up, and current keeps pouring into them, too, they might also reach a point at which they start creating their own internal heat, which feeds into cells around them *and* back into the already-hot shorted cells.
Keep taht up and at some point something gets hot enough to ignite, and a fire started.
Whether or not any of the above actually occurred we'll probably never know, but it's one possible scenario of many.
