I think (My Opinion here) that the helmet is either rolling forward over the head and doing the damage -OR- the helmet trys to come off the head backwards while the head is still moving forward and the bottom of the facial opening is catching the face as the opening falls over the forehead.
I still think there needs to be some development of a CageCap that offers extra protection to the top of the cage to absorb energy and cages have to be opened up and made wider ,to keep the driver away from the cage.