Some pictures showing how avalanches and cornices form
Note the cornice forming at the roof edge. It is caused by snow slippage and wind action. The cornice is very heavy and is dangerous to those below it.
Another cornice forming on a section of roof where there are no roof bars. Some mini avalanches have taken place on the roof section at the left. Avalanches happen when a layer of snow fractures and starts to slide over another layer of snow or the ground. In this case the snow has separated from the roof and is sliding.
A little more to the right on this roof, there is a section with less area and no bars at the roof edge. Here we have a full scale avalanche with the roof now completely clear and with the lower roof, which you cannot see, completely buried.