Slab avalanches are a relatively stronger layer of snow on top of a relatively weaker layer of snow. The weak layer fractures and the slab becomes the avalanche. The weak layer fractures when stress equals strength. Snow can only withstand a certain amount of stress at a certain rate. If the amount of stress is too great or the rate is too high, then the snow fractures.
Wind is the most common cause of avalanches.
Wind can deposit snow 10 times faster than snow falling from storms. The added weight from snow storms also causes avalanches. If the weight of new snow is added faster than the buried weak-layer can adjust to its load, then it fractures and forms an avalanche. Rapid warming or rapid melting of snow can also cause avalanches. For instance, rain on new snow almost instantly causes avalanches.