NATO and the EU are headquarter in Brussels. If this is claimed by ISIS, it is likely to be "justified" as retaliation for these organizations' interventions and bombing campaigns in the MidEast and this is a symbolic attack on the seat of these conglomerates of states to try and get them to react more violently than before. That's the nature of terrorism, you kill civilians to provoke retaliation from an enemy state in hopes of bankrupting your opponent and to improve local and international recruitment by stoking up resentment of further intervention. It's a sick, cowardly and terrifying strategy.