Secutity Check lines at Brussels AIrport

As you say, it was the evening before a three day weekend in peak summer. I'm almost surprised that it was only 1 hour. You can easily get 45-60 minute waits at busy times at the airport.

There was a big police presence in town on the holiday due to numbers expected for the fireworks and bags searched so not just the airport and who knows what intelligence they have also peak time to fly so more people mean slower queues