It happens because somebody is importing django. It might be a third-party library you are using, and it might be a code-path that you are not actually executing (eg: you configured at runtime not to use django, but PyInstaller can't know that).
If you don't need Django, then you don't need the above fixes either, and the simplest solution is to exclude django from the deployment (see the manual on how to modify the spec file to exclude a specific package).
If you are curious you can bump the log verbosity; at some point, it will start dumping all modules that it analyzes, so that you can find where django is being imported.