Brussels has a daytime population substantially larger than its resident population. A very large number of people commute in each morning from Flemish Brabant, Walloon Brabant and further, use the roads, the transport network, the policing and the emergency services of the capital region, and go home to a commune that collects their personal income tax.
The mismatch is structural rather than accidental. Belgian personal income tax is levied where a person lives, and a share of it is redistributed to that person's commune. A city whose workforce lives outside its boundaries therefore carries the cost of the infrastructure that workforce uses without collecting the revenue that workforce generates, and the effect compounds when higher earners are the ones most likely to move out.
Housing is what drives the movement. Brussels property is expensive relative to the surrounding provinces, family sized housing with a garden is scarce inside the nineteen communes, and the regional differences in registration duties and housing incentives make the calculation more complicated than a simple price comparison. Families move out at a predictable point in their lives, and often keep working in the city.

