-Characterizing
When nonhuman head NPs function as objects, Fox and Thompson found that there was a strong tendency for these heads to occur with relativized subjects in relative clauses that characterize or describe (instead of functioning to anchor or ground the head noun):
They’re selling these candies that explode when you chew on them. OS
I don’t like the pants that come down narrow and then bell out. OS
The object noun heads tend to be definite and already grounded in the discourse, so these relative clauses simply provide additional information to characterize and narrow down the scope of the head nouns.