It happens when intranational transportation costs more than international for that resource/location combination

Many nations have minimal tariffs on trade from their neighbors.

For the US steel is something that freely crosses borders. Toronto may import steel from Pittsburgh while Minnesota imports from just across the Canadian border because there wasn't a price premium on it. The end result is that the US both important and exports steel but to different locations

It's all about if getting things internally is cheaper than externally. If that changes across the length of the country then some parts may import while others export