These ne'er-do-wells are at the very least acting the fool if not up to no good. They dislike work, hate tribesmen and their customs, and act highly impulsively. They live for fun – a word which, in this context, is regularly synonymous with attacking the weak and vulnerable. Violence is the answer, whatever the question.
Cohesion is poor and planning, near nil; capable leadership, at best ephemeral. Mortality is extremely high, not only for their victims.
Important to note, groups of brigands will fight each other for any reason whatsoever.
As for ethnicity, one predominates among brigands. The response to 'why are Barbarians so grossly overrepresented?' depends on whom you ask.