I was able to reproduce this, but I am not sure if I can do it consistently. It happened if I performed a reboot from inside of an xterm without shutting xfce down properly (so I didn't use the xfce logout/shutdown/reboot functionality). The next time I reboot xfwm4 did not start automatically. Going into the desktop settings, I had to re-check the "Allow Xfce to manage the desktop".