Sometimes mods can cause issues with the game. It may happen due to a number of reasons: mods may conflict with each other, mods may use outdated textures, mods may confict with the game files, etc.
Sometimes mods can introduce issues to the game, from minor bugs to complete unusability of certain content. This can happen for a number of reasons such as:
Mod is Outdated and/or Broken
Mod is replacing core game files
Mods are conflicting with each other