Comment 3 for bug 222224

oh, well, I found a pretty clear explanation for why this happens. In domainjoin/scripts/ConfigureLogin:

GracefulDbusRestart()
{
[...]
            /etc/init.d/dbus restart
[...]
}

We probably shouldn't do that, considering the dbus package itself doesn't even do this on upgrades. Here's what the dbus package does instead:

    if [ -e /var/run/dbus/pid ] &&
      ps --no-heading -p $(cat /var/run/dbus/pid) > /dev/null; then
        # trigger an update notification which recommends to reboot
        [ -x /usr/share/update-notifier/notify-reboot-required ] && \
            /usr/share/update-notifier/notify-reboot-required || true
            invoke-rc.d dbus reload || true
        exit 0
    fi