Hello & Welcome to JustAnswer. Thank you for your question.
What is the version of your OS X?
Did the issue start after the system was upgraded to El Capitan?
What is the full version of Quark? 10.5.2 ?
Thanks. I need the exact version to check.
Open Quark. Click the Quark Menu from the top left, and choose About Quark.
QuarkXPress 10.5 is not supported on El Capitan, and i am pretty positive that the crash issue has started after the upgrade. I would suggest you download the update and follow the steps listed here and then test. Please read through the information given at the link first. Let me know how it goes.
I have also looked into it and found the same thing what kamil found. This version in not supported in EI captain. For more details you can check this out :- http://www.everythingmacintosh.com/mac-headlines/front-page/important-warning-for-quarkxpress-users/
You can also go through this discussion :-
Let me know if you have any other questions on it.