Wind eroded snow, which often looks rough like frozen waves. Usually found on windward slopes.
Wind erodes from the windward side of an obstacle and deposits on the lee side. We call the eroded snow sastrugi. You can recognize it by its rough, sand-blasted texture. We usually think of wind eroded snow as being stable because stress on buried weak layers has been decreased by wind eroding the overlying snow. Conversely, wind will deposit that same snow on to the lee slopes, which increases weight on buried weak layers.
Sastrugi is not always stable snow. Remember you only see the surface texture. Perhaps the wind only eroded an insignificant amount of snow and a buried weak layer still lingers below just waiting for a trigger. As usual, all slopes are guilty until proven innocent by the usual battery of snow stability tests