For nuance: Belgium, consisting of the Northern Flanders and the Southern Wallonia, is one country, but there are enormous economical differences between the two.
Historically, Wallonia housed heavy industries and was much better of than rural Flanders. This was visible in the way that French was the default language for everyone who was not a peasant, resulting in Flanders (and mostly their population) historically being excluded from higher posts throughout the nation, no matter what field.
HOWEVER, with the advent of service economies, Flanders heavily transformed and successfully became such a service economy, while the heavy industries in the south disappeared almost completely, leaving the region economically (and sometimes physically in terms of pollution, ...) devastated.
Right now, every person in Flanders pays a lot of taxes to subsidize the southern part of our country. We do not have a say in this matter, and most people haven't forgotten that our part of the country didn't get any say when the south was the successful region. Considering we have one of the heaviest individual tax burdens of any nation on Earth (over 50%!), people are understandibly miffed about paying maybe 1K or more in taxes just to prop up whatever failing economy tries to chug along in the south.
Add to that the complete unwillingness of Walloon political parties to even address this discrepancy, voila, you get Flemish-nationalism.
For the record, I'm not one of their voters. But I absolutely understand why they are the largest party, even if they flirt with the extreme-right parties.
The best way to fix this issue was to address the inequality in an honest and correct way, but our politicians have simply refused to do so for generations. It was all but certain it would at some point come to a head.