pretty easy.  basically what happens is you hit the tree, fall in the hole and the snow on the tree that you hit comes down and buries you.  a lot of times people fall in head first, and their skis wedge them in and they cant get out.  then the snow will bury them and they suffocate.  unless you have a friend nearby or can somehow manage to click out of your bindings you are pretty much screwed.  treewells are really dangerous, also you can get the same kind of effect at the bottom of cliffs.