I'm not sure where to put this. It is common when installing KDE from an Ubuntu Gnome base(Gnome,Gnome Classic,Mate,Cinnamon,Unity) to have Nautilus appear at startup afterwards. Basically when the KDE session loads, a Nautilus Window will pop up displaying the home directory. Really you can just close the Window and go about your day, but to stop that from happening...
Go to
System Settings-->Startup and Shutdown-->Session Management
Click on Start with an Empty Session
Click Apply, Close Window
Reboot
Go back to
System Settings-->Startup and Shutdown-->Session Management
Click on Restore Manually Saved Session
Click Apply, Close Window
I'm not exactly sure why this happens, but it will pretty much load Nautilus every time.