Frequently Asked Questions - Session Manager[Xfce Wiki]
Why are some of my applications automatically started when I login?
This occurs when a previous session is restored or the application is set to auto start.
To prevent Xfce from restoring your last session, go to Settings Manager → Session and Startup and uncheck “Automatically save session on logout”
Or use this command to turn off session saving
xfconf-query -c xfce4-session -p /general/SaveOnExit -t bool -s false
You may also need to remove previous sessions
rm -rf ~/.cache/sessions/*