A full account of the incident (in French) is here - http://frenchy.biz/expeditions/avalanche/
PisteHors have a good translation and some commentary here.
The full account is well worth reading and a good reminder to never let your guard down, but here are some pertinent extracts...
At the end of the VB we climbed 200m to join the North West slopes above Chamonix. From a snack bar the route makes a big traverse of the hill through woods. Despite the northerly orientation there didn't seem to be any refreeze. We had dropped our guard, the snack bar and tracks made us think of a ski resort, in our heads we were no longer in a high mountain environment ... There were a lot of people skiing when suddenly I saw a cloud of snow in front, 2 meters high, an avalanche.
Avalanche
I stopped dead with a skid, I just had time to think "I've had a close shave" when the sky fell in on me. I was pushed to the ground, I felt the snow crushing me more and more, burying me alive.
I thought, this can't be happening, I tried to get up and then the snow stopped. I managed to make a bit of space. my head was half out of the snow but I was stuck, I had maybe 30cm of snow on top of me. I'd heard that once the slide stops you can't move, you have to be in this situation to see just how true it is.
...
Six to eight people were now crossing the debris, most on foot, skis or snowboards in hand when Jon shouted "avalanche, avalanche, get out the way!". A second slide was coming down.
...
A ski tour is only finished when you reach the car-park. In our heads we were safe after the glacier.
...
Before each tour make sure you are prepared, that your friends are prepared. Keep your guard up. Watch out for received wisdom, question what you've heard about avalanches. Train. Be ready to make a U-turn, remember that where there is a doubt, there is no doubt. As André Roch once said "it's great that you are an avalanche expert, only the avalanche doesn't know that".
It's worth repeating that the risk of full-depth avalanches is, and will remain, particularly high this season - due to the first heavy snows having fallen on un-frozen ground.
There will be more like this; don't leave the piste without a transceiver (and shovel and probe) and choose your routes with caution.
Stay Safe.