wp-content/uploads folder suddenly viewable

I’ve seen multiple posts about how to keep wp-content/uploads from listing the directory of files to any person who types in that URL. I’m wondering if anyone knows WHY that happens. I have several other WP installs that show a “Forbidden” message if I go to http://[domainname.com]/wp-content/uploads, but one site is showing a directory listing of all files. We looked at a backup of that site from a month ago and it also shows the “Forbidden” message. Any idea why it is suddenly showing the directory? We managed to hide the files by editing out .htaccess, but the page still shows a directory page, it just doesn’t list the files. Weird and frustrating.