Large dry loose triggered from spin drift

While 3 of us were on ascent( about 2/3 height) we witnessed a large spin drift off the climbers right wall which entrained a lot of snow turning into a significant dry loose event. Our 3rd who was a little ways behind got carried to the apron. There was a 2nd party behind us just above the apron that followed us up the couloir. Their crew of 4 was carried as well. The 2 of us that weren’t carried went into search with our beacons and skied down the the apron where we found everyone on top and luckily uninjured.

We never had communication or visual of the other crew on ascent until moments before the incident.  
 

I would classify the avalanche as a dry loose D2. Their was evidence of several small avalanches from previous days in the couloir. Boot pen on ascent was 24-35cm