I don't see a dependent in catalog variables like they have in Choice lists on forms.   What you might need to do is use the catalog UI policies.     Make a different sub-cause for each choice in the cause.   So if you had 5 causes, then there would be 5 different sub cause variables.     Then use the UI policies to hide the different sub causes until the correct cause is chosen.   This sounds messy, so hopefully someone else can come up with a better idea.