There are additional complications at that Bicester junctions, which is why there are frequent smashes there, even before the fog comes down.
Southbound, the outside-laners wait til the last minute to get off, in order to jump the (usually 0.5-1m) queue. Only this means coming to a standstill in the "Middle" lane (just as it goes to 2 lanes), and can't seem to see the danger in a 70mph car ploughing into the back of them.
Northbound, those joining from the A34 have to get in the outside lane ASAP, else they obviously have a small penis. For traffic on the motorway anyway, there is an odd, variable camber as the motorway turns right, then left at the junction. This can cause an avoiding swerve to turn into a slide.
I wouldn't be surprised if it was one of these was the initial cause, then with the fog, pile ups ensued.