It will be interesting to see the rationale from Norwegian's management. Certainly the scandinavian market alone would not seem to justify ordering that number of planes. So it looks like them going head to head with Ryanair (another mega airline with a small local population) in the wider market.
Then again perhaps a degree of skepticism as to whether Boeing and Airbus are going to deliver to their ambitious timescales has encouraged Norwegian to place an each way bet....similar to American Airlines.
Hardly a month seems to go by without some airline that hardly anybody had heard of ten years ago ordering hundreds of jets.