How do long, thin, island chains such as North Carolina's outer banks, form?
The outer banks are, literally, a textbook example of what are known as Barrier Islands. They are thought to be formed through interactions between sediment undergoing longshore drift, coastal currents and seabed irregularities, but to spite a number of theories being put forward since the 19th Century we really don't understand their formation.
We do know that their ongoing behaviour makes them analogous to underwater sand dune systems in that:

the material in them is constantly on the move.
their form is controlled by the prevailing conditions.
they offer a degree of protection to the landscape on their inland side.
and
they are vulnerable to drastic alteration during extreme events such as storms or tsunamis.