why do people refer to some people as rednecks?

They are describing them as low-class, working-class poor.

They’re ignorant and dont fully understand what a true redneck is. Redneck came from people that worked on the farm all day, and their necks got red-ish from the sun. most ‘rednecks’ are posers, because they dont work on the farm or anything like that, instead they’re more like posers, and/or hillbillys.
and f.y.i just because you like country music, does not make you a redneck.

if a white person is “racist” they are called a rednecked cracka.

White people call themselves Rednecks so I have no idea.

The term originated from the red cloths (neckerchiefs) worn around the necks of a certain group of Appalachian coal miners who were violently persecuted, even murdered, by the anti-labor union mine owners in the early 1900′s. It’s a crazy story; one that reveals an America that most people these days would not even recognize. Check out the movie “Matewan” sometime. The term “redneck” has gradually evolved to refer to someone who is from the country and considered “backwards” or ignorant, although some people just use it to refer to people from the country without a negative connotation.