Two shutdown commands in Solaris: One in /usr/sbin, and another in /usr/ucb. If user's path was set to check /usr/ucb when the user simply typed 'shutdown'.
This is correct behaviour for the /usr/ucb/shutdown command because 'y' option is not listed as an option.
Set the PATH variable to look at /usr/sbin first or use full path to /usr/sbin/shutdown to execute the shutdown command.
Additionally, "which shutdown" command can help to find path shutdown command has taken.