There are too many possible confounding factors to say that there is a direct cause. It could be that those who eat more fat also exercise less, for example. So, confounding variables make it difficult to determine a causal link. There would be unlikely to be enough cases at each school to conclude that it was anything else. Just like for the earthquake example in the text, the number of deaths is highly dependent on the density of houses in the area.
Large fires are actually more likely to occur in sparse, wooded areas, where there are fewer people.
Therefore, the relationship may actually be negative, giving the impression that the more acres burned, the fewer the number of deaths. Also, one or two outliers with relatively low acreage but high number of deaths could completely distort the results. There are confounding factors, like more sports cars are red. Also, people who buy red cars may be more likely to speed or drive recklessly. The key is that there are two clusters of points that don't fall into the same pattern with each other as the pattern in each cluster.