Saw this a few weeks back - scary stuff! The only reason i can think this might happen (as clearly all the drivers fail to slow enough) is that they were running to FIA flag rules which dictates you can only have 1 flag post wave a yellow, which must be the post before the accident. (although the clerk of the course can overrule this - that would take time to send a message)
Sounds sensible enough, but say the accident happens 1ft past a marshal post after a corner, this post would show a yellow - not the one before the corner.
I've seen this before at Brands Hatch in a support race for DTM. A car spun and stoped on the circuit at the base of Paddock hill, just past the marshal post on the infield. They threw a yellow flag, but this meant there was no yellow flag on the entry to Paddock - so cars were entering paddock without any yellow to tell them there was a car in the middle of the circuit.
Apparently what is often done in this situation is (common sense is engaged) and the post just infront of the accident will throw a green (when it should strictly be a yellow) - but this force the before the corner to throw a yellow flag.
In the video i cant see where the marshal post is, it could be pretty unsighted and just before the accident - certainly the green flag seems alot further down track.