The author should have mentioned the enormous increase and recent decrease (in Illinois) of NICS checks for purposes other than firearm sales. Non firearm-sales are now more than half of all NICS checks.
I knew that NICS background checks are done on people who want to own a firearm or explosive. (see https://www.fbi.gov/how-we-can-help-you/more-fbi-services-and-information/nics)
You mean that more than half are for explosives, or are there other categories?