How to find out why?
Like any systemd user service, it will be monitorable using systemctl status --user, and its log appear in journalctl --user. If you want to filter for messages from this boot only, journalctl -b --user -u usbguard.
It is supposed to work with wayland. In fact, it shouldn't care at all. It doesn't interact with the display system at all; it uses libnotify to send notification via dbus to the component that displays notifications.