Comment # 2 on bug 1131115 from Neil Rickert

I'm now reasonably convinced that the shutdown of nfs automounts is due to
"gdm" deciding that it is time to suspend the system.

I turned that off in "gdm", using the command suggested by the Arch wiki:

 sudo -u gdm dbus-launch gsettings set org.gnome.settings-daemon.plugins.power
sleep-inactive-ac-type 'nothing'.

I have not seen the problem since making that change.  And some updates have
taken long enough that I should have seen the problem if it had not been
solved.