The lad in question lost it on the climb up from Eau Rouge to Radaillon and hit the barriers. In itself it wouldn't have been a major issue, but it was only the second lap of the race, the cars were tightly bunched and accelerating up the hill. Car rebounded off the barriers and broadside into the oncoming traffic. The lad who t-boned him had nowhere else to go.
Absolutely horrific incident.
Cuts me even more as I spent five years doing strength of materials research in a bid to make single seater cars more resilient to side impacts. Alas, at the speeds involved in today's incident, even with the best will (and chassis construction) in the world, those kinds of impact are not survivable.