That's a really rudimentary explanation and a situation that doesn't always cause a crash, but you get the idea. Skyrim then loads A, and B, and when the assets for mod A are replaced by those from B during startup, the game crashes. Mod B replaces Pinewatch with a 12 story wizard tower. It mods Pinewatch in a way that makes sense for the mod. Mod A adds NPCs, and homes them at Pinewatch. This is why the order of how your mods load in Skyrim is important, as well. It means that one or more of your mods is malfunctioning, usually because of a conflict between two mods.
