Your shutdown string is not the default for Windows 8 and onwards.
The OS will use /hybrid which is used for fast startup shutdowns. Start8 will use this flag in order to match up with the OS. Some machines do not like this option due to bugs in their BIOS and you can disable this setting using the OS setting to disable fast startup. I think this is in power settings.
You should experience the very same hangs when shutting down from the charms menu. Be aware that in hybrid mode the machine can take some time after the monitor has turned off to actually turn off as it is saving information for when you bootup.