That was the "Marian" ones. People with way too much cash and ambition were raising and disbanding them all the time so there generally wasn't any shortage of out-of-job soldiers looking for a new employer, I've been given to understand.

The "premarian" levies were normally mobilised on rotation AFAIK, though I don't know how they handled geographic distribution. Would seem to make sense to take as much of the manpower as possible per batch from a given area, so the soldiers are already more or less familiar with each other (and regional dialects, quirks of custom etc.) and thus "knit" faster, but OTOH that's also "putting all the eggs in one basket" as casualties go - a mauled maniple would mean an entire village or small town losing a major chunk of its adult male "rural middle class" populace...