Lazy Boy Avalance

2 of our party skiied the slope without any indication of instability. 3rd skier entered the slope slight further skiers left, closer to some rocks and triggered a slow moving avalanche. Crown was about 20m above the skier, varying in depth from 10-60cm, with a few different layers failing. Skier was able to ride out to the side. From below it's obvious to see the feature and how the convexity and rocky outcrops could cause weakness. Not so obvious from above! All party are fine.