Well, in the same way that if someone is shot in the chest, the cause of rapid bleeding out might be the big blood pumping organ in the area.
No, put finding where that first movement happened is perhaps key to finding the hand that flicked that first grain, and does bear the whole blame.
> Your example is of relatively stable system (presumably healthy human life)
We don't consider the healthiness relevant for most legal purposes; are you familiar with the eggshell rule?
That's the problem, is people aren't looking at the actual problem. Fires are natural for this area, your house is not.