Usually a combination of the Apache index pages, dumb server setups, and being people being lazy, careless and/or forgetful when they dump a database to disk.

Usually a combination of the Apache index pages, dumb server setups, and being people being lazy, careless and/or forgetful when they dump a database to disk. Also, sometimes an index.html file is accidentally removed, causing (brain-dead installs) to suddenly reveal the contents of a directory, eg. http://shahinfosoft.com/ http://shahinfosoft.com/