I believe that the most often used POD here is OTL king Willem II dieing at Waterloo.
Actualy there are various way of avoiding it. Best way is to simply treat the Belgians better. Other ways are to keep Willem II away from Brussels and screwing up. Or faster intervention by the Dutch king in the early stages of the revolt. Or France does not get involved. Or Prussia/Austria/Russia support the Dutch during the revolt. Probably a couple of others.
Actually one can argue about the general treatment of the Belgians. Yes Willem I made mistakes, but OTOH the population in what would become Belgium and what stayed the Netherlands was not generally treated differently. For instance any argument that Willem I was somewaht autocratic applies to the entire kingdom. Not to mention that Willem I championed the industrialization of the Southern Netherlands (later Belgium); in fact he still owned a large share in the Societe General after Belgium was independent

. Or that the industry of the Southern Netherlands profited from the colonies, which were inherited from predecessors of the North. OTOH the North, which was focused on trade, preferred free trade, whereas the industrialized South seemed to prefer protectionism.
In fact in some regards the Flemish region was better of in the kingdom of the United Netherlands than they later were in Belgium. For example education in Dutch. This was however also opposed by the Francophone Flemish elite, even though the majority of the population there did speak Dutch (dialects).
Anyway yes giving the Southern and Northern Netherlands equal representation, even though the population in the South was larger was unfair; however OTOH any reversal would also be bad. So yes, more Southern representatives would have been fairer, but they should never have been that large that they could just impose their will on the North.
Maybe
The Southern Netherlands (later Belgium) had to share the burden of the debt of the former Dutch Republic (Batavian Republic and kingdom of Holland), which in an union would have been unavoidable. IIRC Belgium even had accept a share after their independence.
Willem I also didn't handle the Roman Catholic faith quite well, though that is broader than just the Southern Netherlands, the North also had a sizeable Catholic minority (in some parts of the North even a majority).
Catholics also were underrepresented in many institutions, but that was a bit of a two way street. On the one hand there might very well have been some discrimination, but on the other certain Catholic bishops also discouraged Catholics from working for the regime.
Anyway once in South the Catholics and Liberals managed to agree upon an unlikely political alliance Willem I was in trouble. That combined with the 'bad luck' of a famine, which proofed to be the final straw.