I was just talking to someone about this. Worth remembering that an autumn snowpack is a thin snowpack. The ground is probably still quite warm (even in Scotland, I suppose) so you get quite a strong temperature gradient whenever there are any clear nights. Facets and depth hoar can form very quickly in these conditions and when buried by some cohesive snow you can see the results. The message is “beware the (wind) loaded 30 degree+ shaded slope”.
There wasn’t a huge amount of snow mobilized and the run-out, bar the rocks, looked reasonably benign. The debris must still be several tonnes of snow, enough to bury you in a hollow if you were caught. The slope must be around 35 to 40 degrees at its steepest. Convex entry and, though it is hard to tell, some cornice formation.
I think this will be one of the human factors and ignoring obvious signs. They noted the wind transport in the parking lot, that was maybe a clue to avoid wind loaded slopes. They seem to be well equipped and use some technical sounding words which would make you think they were experienced.
It is similar to Jerome Buc’s experience
http://www.skipass.com/_videos/avalanchepiau.mpg
Jerome was dragged over rocks (there was little base) ripping his ski pants and leaving him with lacerations to his buttocks.
