it creates a big-time avalanche risk because the heavy top snow moves and slides in slabs on top of the icy lower layer/crust. Once the avalanche starts with the slabs sliding, gravity takes over and everything picks up momentum as it moves down the mountain. Anything in its path (trees, humans etc...) get carried with it and only rarely survives.

Having grown up in Alaska and spent a lot of time in the Alaska and Idaho backcountry and been part of avalanche search and recovery efforts, in my opinion, an avalanche death has to be one of the worst deaths imaginable.

Short answer... massive avalanche danger conditions that aren't worth the fun of what you do on the top (ski, board, snowmobile etc...)