Upon fresh install KDE will restore the windows and applications that you had openend during the last session. The idea behind this is that it allows you to quickly continue where you were left and provides a similar workflow to hibernate or suspend. However, the main difference with its implementation is that many applications won't properly restore the documents you had opened and might start entirely blank.
If you don't like this behavior you can change the settings to let KDE start with a empty session:
- System settings
- Startup and shutdown
- Session management
- On login -> Start with an empty session