We propose a new hypothesis that supports the theory of a blitzkrieg waged by human hunters against some species of animals. We suggest that some species were doomed when encountering human hunters because of their tendency to live in large flocks. We introduce a mathematical model that, at least in theory, shows that animals living in large flocks during catastrophic times are more susceptible to mass extinctions than animals living in small flocks. We also show that in non catastrophic times aggregation may help survival.