Why does my character and some NPCs fall through the ground/floors? Just curious.

Usually it's because of mods.

However, the game is made up of Textures and Meshes. If there is a point where the two meshes are not quite seamed up, then you can drop through into the void under the map. It does not happen very often and you have to hit a specific spot in the cell to make it happen. A mis-rendered cell can also cause this to occur to, but that can be fixed with a reload.

it's a bug, i've had it happen in games like gta san andreas (from rockstar games), and elder scrolls oblivion (for the ps3).