Well, it could be viewed in a few ways..
An neighbour is not responsible for an accidental fire which crosses boundaries.
But a neighbour would presumabely be liable if they installed a Christmas decoration on their roof which blew off and smashed through your sun room.
If one wall fell on another it should be obvious that the wall on top fell on the one on the bottom. .. although it's concievable that the wall on the bottom fell first, and the other wall just happened to fall on an already collasped wall.
I wonder what would happen if a tree fell on your wall, or your car... any comeback against the tree or land owner?