Read through the boot scripts in /etc/init.d/. If you use the _netdev mount option, a script can run "mount -a -O no_netdev" to mount filesystems that aren't net devices, and defer mounting them until later. It isn't an option on how a filesystem is mounted, but used by scripts to control what gets mounted. Try grepping for no_netdev in the scripts.
I wish to pass a key file to the crypttab from an NFS mounted location.
Having the secret on an NFS share defeats the purpose of encryption. It will only protect you when you dispose of the hard drive. The passphrase should be committed to memory and not exist in a file, unless you have it locked in a safe.