I've placed the suicide possibility myself, but dismissed immediately.
One possible and strong explanation is that the civilians became trapped by something or someone, and the fire was set by the French, as in unumerable villages in that area (French, Portuguese and British all agree on this MO - the burning, not the trapping, just to make myself clear). Also, a good possibility is that the French were presumably the ones who trapped the civilians by whatever means.
If not, and if the fire came from other buildings adjoining, the civilians would have plenty of time to get out, even if the French were still there. Something or someone must have made them stay, either physical impossibility or a clear threat to their lifes outside (clearer than being devoured by fire).
Jorge