Thing is, it's not for "no reason", though I understand what you mean. The reason is that people are following a toxic and violent ideology, but there are many in the West that are too afraid, too deluded, or have been cowed from talking about it openly. Which of course only ensures that these sorts of attacks will continue.
I will be a little bit more radical, but I think that some states in Europe should bring again executions. France, for instance, used to behead people. ISIS are keeping hostages? Ok, let's do the same with their people. I really doubt that the intelligence services really can't find out about the radical muslims living within the Europe's borders. Now, I'm not saying to arrest all of them, to kill all of them, but rather to cut down some of their lines.
Belgium is part of the coalition against ISIS. And since they are the most vulnerable out of the bunch, consider Russia and the US, they were targeted by the terrorist group. At least, that's what I think of. Plus, security in Belgium is not that tight and there are lots of jihadis in there as well. It's very easy to target the country. There's also this theory that since they have caught the last attacker on the Paris incident, ISIS has had to move and make an attack. It was said that the supposed attacker was negotiating with the authorities so yeah, that might be another reason for the attack. It is a terrible incident and I truly sympathise with the families. That's why it's important to stop ISIS once and for all.
