A southern contraction turned into a pop culture slang. To people in the south "yall" is used to distinguish themselves as southern so not to be confused with New Yorkers or otherwise "Yankee bastards" despite the obvious distinction in accents. In the north "yall" is used as a comedy staple to stereotype what yankees refer to as "idiots" and otherwise "ignorant rednecks."
In popular culture, "yall" carries the same stereotype as the northern taboo only it grants several comedians the ability to mock and otherwise satirize their own culture such as Larry the Cable Guy and Jeff Foxworthy.
"Yall wanna come to Herbert's field and go cow tippin'?"