Spa-Francorchamps is notorious for weather differences that occur on different parts of the course. That, along with the high speeds, make it a very dangerous venue.
It looks like at least three of those cars took almost an identical line into the barriers. Sudden change in something on the track must have gotten them all.
A classic example of target fixation. It reminds me of the student driver who loses control, runs thru a fence and hits the only tree, which is out in the middle of a 20 acre field.