Sorry if this kind of question isn't allowed here, but I'm at my wit's end. VB randomly started launching on startup about 2 weeks ago (on EndeavourOS) and I can't figure out why. There's no shortcut in ~/config/autostart, it isn't in the KDE startup apps list, and I can't find anything virtualbox-related with systemctl either. There's also no setting in the VB app itself. WTF?
Try to exclude it from being restored on KDE startup. There could be something running in the background that keeps getting saved and restored. Go to System Settings, System, Desktop Session, put virtualbox in the ignored applications list. You can also just choose "Start with an empty session" and then relog to see if that stops it from coming back, then you'll know if this is causing your problem or not. Good luck!
Unlike Windows, in Linux every process has a parent except for PID (Process ID) 1, which is the init system. You can run a command like ps auxf and you'll get a tree of all your processes and you can hopefully see what spawned VirtualBox.