Youll find several varying sources , each more believable than the last(if accuracy is important go read some label on a can of soda).
The North Carolina Covenanters of the Presbytyrian church wore red strips of cloth to show their dispkleasure with a church edict
The Mine Workers Union before the Blair MNountain Massacre in the 1920's also wore red kerchiefs around their neck.
No matter, its a derogatory term for a (usually) rural poor person with coarse manners. (LAter extended to mean racists also).
Rednecks like grits, moonpies, RC cola, Hemis, anything that Jeff Foxworthy says,Marlboros and NASCAR. Most will also fly the first CSA battle flag in the hole where their car antenna used to be .