Vullaby are too young to hunt for food, for their Mandibuzz mothers (or their trainers) typically feed them more than enough, and should they perish the Vullaby will surely starve, yet they chase smaller, weaker pokemon anyway. Some have seen in this behavior an atavism from Archen or a more recent flightless ancestor, others a developmental phase which uses many of the same muscles as finding prey through flight – although the targets for a fully grown Mandibuzz are already dead. Although the debate remains unsettled, mythology offers a far more interesting explanation.
According to legend, Vullaby chase the weak because they carry the instincts of a predator, and their transition to scavenging occurs only once they have learned the futility of the former task. Mandibuzz encourage their Vullaby to hunt not in the hopes they might supplement their diet by succeeding, but because they know they will surely fail, and that through that failure they will learn how much easier it is to scavenge instead of wasting their time hunting once they are old enough to actually catch and kill prey. Despite being dark types, they claim, Mandibuzz mothers have an affection for all life, and they do not want their Vullaby chicks to cause any excess death, especially in a way which expends too much energy for it to be worth this species' effort.
In reality, even Vullaby raised from a young age by human trainers quickly learn the lesson of scavengers. The closest thing to a meaning any researchers can agree on is that Vullaby simply seem to enjoy scaring other pokemon, whether by imitating a predator's chase or by plotting more garden-variety tricks; should they literally scare one to death, no Vullaby will mind getting a free meal for their trouble.