I think it has to do with the way the Haskell packages are disturbed. So if you installed the pandoc from extra repository, and there was an update for any of the Haskell packages that pandoc depended on, then pandoc will have to be updated again. Meaning, instead of pandoc forcing Haskell updates, it is the other way around.
If you only use pandoc and don't normally program in Haskell, check out pandoc-bin. This way you will only get updates for pandoc itself and its dependencies won't force an update.