The accident happened in Walibi Holland in the Netherlands . The track is closed until it is approved that it is safe. Altough we don’t know for sure, I think it spinned around due to a sudden movement and centrifugal forces. If that was the cause they can improve it by making the car heavier (and more stable) or by preventing that the user makes sudden movements by placing a rotation speed lock.
Here you can read the Dutch story.