The characters are given the task of transporting a flask of highly volatile liquid a long distance. The flask cannot be shaken too much or it will explode. The adventure involves stormy sea-voyages, bumpy cart rides through densely populated towns and horseback combat. In short there are many opportunities for it to break and explode.
As population increases, animal souls are promoted to human before they are spiritually advanced enough in terms of impulse control, etc.: this shouldn't have happened, but karmic progression is automatic, after all--it's a force of nature. But the 'animalization' of humankind bodes ill for civilization.
No doubt the same goes for elves, dwarves, and so on.