What we know about the place is that it's a single underground city. Odds would seem to be that the planet is uninhabitable for the most part, then (no mention is made of lost surface settlements or the like). Does that make it dependent on outside help? Apparently not, as the city is still working after having severed its links with the outside (no mention is made of non-UFP links, either).
But building an underground city on an otherwise lifeless planet suggests one of two Trek staples: that the planet had something of value, meaning it would be something of a banana republic, ruled by those in charge of shipping out the product and shipping in the riches - or that the planet was a refuge for some sort of an isolationist cult. Either of these monocultures would be susceptible to instability of a single-point failure sort...
By the time we meet Turkana IV, there's no sign of exports or cults (remaining?). It might be Turkana had solved its problems by getting rid of such inherent weaknesses, then. But "getting rid" might have required drastic measures, ones incompatible with UFP oversight, which is why the place seceded.
Timo Saloniemi