I was stationary right next to a multi car pile up in similar circumstances on the opposite carriageway on the M4 near Heston years ago. Its the scariest thing I've ever seen/heard/experienced. Very quiet, then a screeh of tyres and a horrible bang, then silence again for a few moments and then it happens all over again. What made it worse was quite a few fires started and you could hear a sort of whump sound when they started and then see an orange glow.
That was in incredibly thick fog, which was why my, much less busy carriageway (west bound early morning), had ground to a stop.
I can't really comment on the wisdom or otherwise of drivers, but I'm pretty sure nobody intentionally made this happen. I reckon its a lot like the accusations of rubber necking you hear about all the time. actually the Highway Code does suggest you should slow down and take appropriate action when you see blue flashing lights. When you do you get accused of gawping. I suspect that some people slow down more than others in fog, and the disparity in speeds is probably the problem, especially so when as in this case you are near a river. You often suddenly run into much thicker fog. etc etc
Its just not always as simple as someone is to blame and someone is a **** when things go wrong IMHO.