Awesome! This is so rare! The Netherlands have fo- wait...what?
This happened in Death & Taxes (which quickly became one of my favourite mods, due to the empires actually getting quite nice borders, and countries forming more often, though this is a very wierd case), and I have no idea why. The Netherlands didn't actually form, but seem to have rebelled from someone. How they got the core is something I'm still puzzled about, the only other core they have seems to be in Ghent.
Probably, what happened is that a foreign power owned the province, then Dutch nationalists appeared and took the province. If, at the time, all the other Dutch-culture countries existed, then Netherlands would have been the next choice. And when nationalists take a province/provinces, the new country will get cores on all of those provinces.