They claim to be making a film for the county. It would seem that the authorities want to document the true scale of poverty in the locality, and are using poor families like Granddaddy Cain's to illustrate the point. Unfortunately for the hapless film-makers, neither Granddaddy Cain nor his wife are willing to play ball. They regard these intruders as a total nuisance and want them out of their hair as soon as possible.
The film-makers seem to be operating on the basis of the racist assumption that, because this family is African American, they're on welfare. Whereas in actual fact, the Cains are a hard-working, self-reliant family who don't take kindly to being portrayed in such an unflattering light. So it's not surprising that Granddaddy Cain smashes the men's video camera and sends them packing.