We (a party of 5) started up in the morning headed up the west side of Box Elder from the Dry Creek trailhead with the intend of skiing southwest off of the summit and then down through mellow WNW facing aspens. I've done this tour that last 3 or 4 years on New Years day as it is generally safe and has an old road to by pass the scrub oak even in low snow years. However, since we knew of a party skiing the same route yesterday, rather than taking our normal route up, we instead opted to save time following their skin track one ridge further north.
Once we reached 8000', we were already experiencing quite and bit of wind out of the East and we decided that a summit descent wasn't going to happen and that we'd just go to timberline and ski down through the aspens. The track we followed also was definitely not better than the normal route, so we were moving more slowly than expected. Between us and our descent of choice was a fairly open, slightly brushed WSW slope that yesterday's skin track crossed. Open seeing the slope, I noticed that a gully was being cross loaded and about a hundred feet below the skin track a small (2-4" by 10') pocket had pulled out. The skin track crossed the gully higher and was safe (there was a smaller 1" deep pocket above a flat that cracked when I hit it with a pole). We spread out and continued, wary but not too concerned. I had briefly considered putting in a new track up to the ridge to avoid crossing the slope, but didn't say anything (wanting to get out of the wind quickly) and continued across.
A few hundred yards latter, I spotted a couple of obvious pillows. The left was 30' wide by 20' tall, and the right was 40' wide by 20' tall. The track went below the left one with plenty of breathing room both towards the toe of the right pocket. When I tested it with a ski tip, the perimeter of the pocket cracked all the way around but did not slide. Their appeared to be 2"-10" deep pencil-hard windslab. On the other side was sheltered, low angle terrain. I stopped and waited for another member of our party to decide what to do, not wanting to cross the pocket that had cracked. We agreed that it would probably be okay to carefully skin between the left and right pockets and then cross over the top of the right-hand pocket were it had cracked out. I started up but by this time the other members of the group had nearly caught up with us and we didn't realize that they had just moved below the left hand pocket.
After a couple of narrowly spaced kick turns, the slope above me and the left-hand pocket all slide. Where I was, the slap was only 3-4" thick and I stayed put (although it packed some punch and I was not certain I was going to stay were I was). The partner I had consulted with was directly below me and also managed to arrest before going anywhere. The pocket to our left, however, turned out to be tucked in under a small rock, allowing the slab to be 2' deep at that point (much to our surprise). The large slab went directly towards the other three members of our party. One managed to get out of the way, another was carried a few feet, but the third was hit directly and carried about 40'. He managed to grab a 3" aspen which promptly broke off in his hand but slowed him enough for the slide to go by. At the same time, I was able to quickly kick turn and move into the bed surface to maintain visual contact.
At this point everyone was okay, but the wind was still blowing hard. Three of us retreated to slightly safer ground while another went down to help search for a missing ski. After ten minutes of searching we found nothing and didn't feel comfortable lingering so we retreated minus one ski. One think we noted afterwards was the chaos associated with the slide. While I was ready to dive down and start digging if necessary, the un-caught member of our group didn't know that anyone had taken ride - we would have been wise to take 30 seconds to communicate before taking any action.