While ascending this steep slope a snowmachiner triggered a large hard slab avalanche that stepped-down into old weak layers of the snowpack near the ground. This avalanche occurred on a northwest aspect at 3900 feet with a slope angle of 41 degrees. The crown was at most 7 feet deep and 175 feet wide and ran 350 feet downslope. As the rider climbed and traversed across the slope they impacted an area where the snowpack was shallower and weak persistent grains existed more near the surface. In this location we found the lower 2/3 of the snowpack to consist of faceted grains which were transitioning from a rounded moist facet back to a dry weaker solid facet particle. This cross-loaded slope has a concavity where old wind blown snow had drifted forming the thick dense layer which released. Luckily the rider was in the right trajectory which enabled them to escape the avalanche and retreat to more stable snow. Previous tracks were in the vicinity but none had been placed on this slope and this area had not previously avalanched. We did observe other avalanches in this area that were of similar aspect and characteristic, some being higher in elevation and further up valley.