I suspect they are nomads who used to protect the salt sea route, absorbed some of the fleeing nomads, and then some part broke off to come here and continue protecting the trade route between east and west, because it's incredibly lucrative.
Possible that they showed up to see why the porcelain and dye trade cut off. A lot of nomad groups really DO want to settle, but all the good land are already taken and fortified.
I doubt they absorbed much of the fleeing tribe though.
They'd have had worse disease problems if they did.
It really comes down to what the Trouble mechanic represents. Considering that it is only present in free cities, rather than all true cities, it would seem that it has little to do with low-level criminal activity, or even urban unrest in general. Instead, it has something to do with a free city's increased autonomy. I would hazard a guess and say that a free city with colossal walls could quickly subvert the local military forces to their cause instead of the nation's. With a strong defensive setup, independent military forces, and relatively strong economic influence, it would be fairly easy for our free cities to start making demands from the rest of our polity.
Noting that Panem gives us a bonus to trouble rolls.
The classic Free City problems are:
-Cultural friction. Under their own internal management and with a high population that never sees anyone else but each other, Free Cities are cultural reactors which generates cultural friction with the countryside(you see this even today) due to changing all the time
-Political friction. The governor of Free City Redshore no doubt will have friction with the governor of Province Sacred Shore because of jurisdiction disputes where they while the provincial governor has greater power, the city mayor can overrule him on local matters.
-Economic friction. We already see this already. The cities have an insatiable demand for metal, fabric, dye and manpower, which means very different economic drives.
-Corruption. The governor controls corruption a lot, certainly, but also has enough latitude to set city ordinances which further formalize organized crime(or lobbyists, it's really hard to distinguish the two at our level of abstraction). Panem helps here too, the lower classes don't have the kind of incentive to crime that concerns with subsistence could inspire.
But, while the Urban Poor are fed at state expense...well there is incentive to maintain jolly cooperation
Fortunately, our panem policy has increased the dependence of the free cities, and we do have a decent intrigue score to warn us of any problems. I would still feel much more comfortable if we had enough mercenary companies (with 4-5 loyalty) that we could theoretically lay siege to all of our free cities at once, while still having the rest of our army free to deal with potential other issues*. Normally having that many mercenary companies would be prohibitively expensive, but each marketplace can pay for a mercenary company all on its own.
This part here meanwhile is completely wrongheaded. Free cities do not tend to present a credible military threat. They depend too much on the rest of the country that they can't really expect to win anything , though they can and do try to protect their interests. You will never see multiple free cities need suppressing at once unless the whole country is about to go like a powder keg.
Especially when the cities don't really see the provinces as rivals(after all they don't make the same goods), but DO see each other as rivals.
