Uthgart barbarians are the decedents of the survivors of Nethril. When the Netherese way of life came crashing down the commoners and slaves of the empire decided to rid themselves of all elements of Netherese culture - living in cities, practicing / studying magic, complex forms of social hierarchy . . . "see where this way of living lead you"?
Eventually Mystra persuaded some of tribes to accept sorcery as a weapon and the newly formed god Uthgart found a way to weave magic into the barbarian way of life so that the nomads weren't wiped out by their city dwelling neighbors.
Similar story with Minsc's people - decedents of the slaves of a corrupt, demon binding empire. The terrible secret that Dynaheir and her sisters hold close is that their secret society is the last remnant of that evil empire.
