After re-watching Arnoux's contact with the rails it seems that may be something went wrong withe the rear right suspension before. Not sure though…
What is seen on this video
https://www.youtube.com/watch?v=RvNlhFmNmTM
is that the car is nervous, not very well balanced, and has few brakes. What is obvious is that Rene is unable to do a proper double de-clutch!