It looks like the plane which lost cohesion was the ground/snow interface. This would imply that the ground was warm enough to melt the snow touching that interface. The snow doesn't look deep. I wonder if they have had the same kind of winter we have had in California (very warm and little rainfall). I don't under speak French, but I think I heard "very impossible" by one of the on screen persons.
I thought avalanches were supposed to be so fast...seems kinda slow to me.
I would guess it doesn't seem slow when you are caught in one.
That looks like a climax avalanche to me; all the way to the base. Extra warm temps were mentioned, including melt water perc'ing to the base. The amount of force to take out those pylons is serious. Lucky that no one got hurt, I hope.