Why, when I delete some files in Samba, does it not delete them but instead merely change their filename from filename.txt to ._filename.txt?

This is not the behaviour one would expect when "deleting" a file, so I'm wondering if there's an option I forgot to set somewhere in the samba config.

It does this to some files but not to others, I have not yet spotted a pattern to its choosiness. There is a Ubuntu 12.04 machine and a Mac OS X machine which have write (and thus delete) capability, no Windows machines have write permission.