Can anyone tell me the origins of the custom of ...

To be honest I don't like cheddar cheese, so I have to admit this is one tradition I am not familiar with. Fruitcake became a symbol of the good crops, where you use lots of nuts and fruits, and celebrate next year's produce with last year's crop. I guess cheese and fruit go a long way back anyway, and the cheese enhances the flavor of the fruit and vice versa.

I bake Fruitcakes each year.  Although I cannot tell you how the practice of eating cheese with fruitcake orginated, I can provide a brief history at this website www.hungrymonster.com/FoodFacts/Food_Facts.cfm?Phrase_vch=Fruitcake&ttl=

Also, you may want to ask your family members if the act of eating cheese and fruitcake is done in your home or in your local.  Perhaps someone you know started this new tradtion, that I've never heard of. 

I have never heard of this and beleive it or not I am one of those people that actually like fruitcake. It must just be a regional thing. Sorry.