Any version of QuarkXPress prior to version 7.31 is not compatible with Mac Leopard operating system 10.5. If you are running any version of QuarkXPress 6 on Mac 10.5 you may notice some odd behavior. QuarkXPress 6 is only certified to work on Mac OS X v10.3 and 10.4 (and Microsoft Windows 2000 or Windows XP).
I would suggest upgrading to QuarkXPress 7.31 or later for compatibility with Mac 10.5.