Multisite appears to be creating directory

I have WP multisite set-up in root directory (public_html).

I have created a site called “stamford”.

Every few days, I discover that there is a directory “stamford” created in public_html, meaning that anyone trying to access the site will receive a “directory of /stamford” page instead.

Can anyone tell me how this directory is being automatically created and how to prevent it?

Thanks!