A problem for flat roofs is drainage. A roof with curbs and roof drains can quickly get very heavy.
When roof sumps get clogged with snow that turns to ice, and then it rains, the rain has nowhere to go and the roof becomes a swimming pool. This can happen with leaves too.
Water weighs 8 pounds per gallon. 6" of water on a roof weighs 26 pounds per square foot. Add the snow and have an area where drifting produces an extreme point load and you have failure. Codes try to anticipate this, even going into requirements for parts of a building where drifting can occur. But you only need one member to fail, to bring things down.
Another problem can be wind. Wind can create differential forces on a structure, pushing on one part and creating a vacuum on an adjacent part. And wind combined with snow......