[Jaroslav Černý]

There is no pretending, there are people around that were in that race, saw the incident, were part of it or saw it on replay.

Pretending then example situations: the car ahead had their left clear completely not only "completely" (where half of their cars overlap) and did not push/bump car on their left then there would be no collision and as long as both drivers adjusted speed down accordingly to racing situation there would be no contact on top of the hill either and both would exit onto the straight just fine.
Obviously when cars do overlap before corner entry, one or both do not give the other space and contacts happen then they both usually go out.
Another issue on Eau Rouge is rejoining the track especially rejoining on it's upper part or behind the hill in blind spot, do this wrong and it's a carnage.

Overtaking in Eau Rouge? Quite doable when drivers respect each other and give enough space. But when one can't do it, thinks the other can't do it either and thus tries to block or otherwise not give space then contacts happen... it's a crash.

https://youtu.be/SnOOqY8huvA?t=2m5s