Internet Information Server (IIS) first treats SubDir as a file that it should give back to the browser. If this file cannot be found, IIS checks to see if there is a directory with this name. If a directory with this name exists, a courtesy redirect with a 302 "Object moved" response message is returned to the browser. This message also contains the information about the new location of the directory with the trailing slash. In turn, the browser starts a new GET request to the URL with the trailing slash.
With this slash at the end of the URL, IIS treats this as a directory and tries to obtain either the default document from this directory or, if no default document exists, returns the directory listing or a "Directory listing denied" error message.