Brussels is the capital of Belgium and also of Europe. So this is the ideal place to express dissatisfaction, read: to argue. This is where all the official institutions are located, and in addition: where the money can be obtained.
To give you an idea: in 2019, 1085 demonstrations took place in Brussels!
https://www.bruzz.be/samenleving/voor-he...0-01-30
There may be many participants (+100k) or not many (100). Noisy or quiet, peaceful or unfortunately, sometimes violent.
In this case, farmers were asking for more money (subsidies) from the authorities. The demonstration turned out to be violent. A tram became the victim.