http://cgit.freedesktop.org/NetworkManager/NetworkManager/commit/data/NetworkManager.service.in?id=1d89bc0004ec27fbc0c89f17861118c78d7eeab5 and the referenced bug (https://bugzilla.redhat.com/show_bug.cgi?id=876218#c49) have a very nice story why NM is not tearing down the network on 'systemctl stop NetworkManager' => on a shutdown, if you have a NFS mounted root, the root 'disappears' on you and the shutdown hangs...