Skiing (1x boarder, 1x skier) in bounds at Stevens Pass, hiked Cowboy Ridge, traversed along the ridge, dropping down into the horseshoe cliffs area. We both traversed on an existing ski cut on a ~37 degree slope (we had watched two others ski this exact line the run beforehand). First person dropped down into the apron, took a few turns and stopped, at which point the hillside broke away, sweeping the boarder downhill and underneath the snow. He somehow popped to the surface behind a large tree, at which point the remaining snow and sluff continued downhill into a treed chute.