Problem: An Ubuntu 16.04 LTS based distribution reboots when you request a shutdown.
Solution: Enter the BIOS and disable "Wake on LAN".
Circumstances I encountered this problem under:
Freshly installed and updated Xubuntu 16.04.1 LTS on a Lenovo M93z AIO PC. No Ethernet cable plugged in, wi-fi active (WPA2-Enterprise/PEAP/MSCHAPv2) on shutdown request.
Network adapters in the system:
Wireless: Intel Centrino Wireless-N 2230
Wired: Intel Ethernet Connection I217-LM