From what I can remember,the amount of foreign material in the air such as dust.. etc,and possibly the internal effects of infection,medications,allergies,things of this sort,all contribute to the development of ear wax.
In my life I have seen this to be true.While taking certain medications my ear wax production seemed higher than normal.Also while working in a high dust area(my shop) makes the inner ear make more wax to protect the inner ear from the extra foreign materials from infecting the inner ear.
This site might answer this clearer for you.
The Ear Wax FAQ