Just as we speculated. Heavy loading on weak facets.
SAC - 12/12/09
Deep slab instability is expected to occur in below treeline areas as the October 19 facet layer and possibly the November 22 thin rain crust/facet layer become overloaded by significant accumulation of higher density new snow during the next 24 hours. A variety of weak layers are known to exist near treeline and below treeline in the lower and middle portions of the snowpack near and above 7,000' on NE wind protected NW-N-NE aspects throughout the forecast area. These weak layers vary by location and individual slopes, taking the form of either basal facets (Sierra Crest and Carson Range), facets on top of the early October basal ice mass (Sierra Crest and Carson Range above 8,600'), and as small faceted crystals on top of the thin Nov 22 rain crust (Sierra Crest only up to 9,000'). Stability tests and fracture propagation tests performed over the past several days indicate that significant avalanche activity is possible below treeline once these weak layers become overloaded and snowpack failure occurs. This is a situation that is unusual for this region. Backcountry travelers cannot afford to be lulled into a sense of security below treeline at this time.
Be careful everyone.