The NYC music blog Brooklyn Vegan has a Google-translated statement from the Pukkelpop organizers, including this statement:
What happened here is exceptional and has been very unpredictable
But check out this comment from one of my readers, "Festus", in germany:
your guess is right [about a storm like this showing up on radar]. i haven´t seen the belgian radar, but i´m just 200 km northwest in dortmund, germany. the stormfront was clearly visble on radar, way before noon german weather services predicted Xaver hitting us from 20.00 to 23.00 and that´s exactly what happened. a tent-based festival decided to close down and sent his visitors home some minutes later.
Sigh. You would think, after the horrific tragedy in Indiana, that the organizers here would have been even more cautious than normal (although this is a different festival with 10's of thousands of campers). We had several severe thunderstorms last evening here in NYC, and I was watching the line that eventually hit us for literally at least an hour. Once in a while, storms can pop up or ramp up in intensity quickly (as apparently happened in Joplin, MO), but even in those circumstances weather forecasters are very good at predicting when conditions are favorable for this kind of storm development.