Why do people in South Vietnam call Northern Vietnamese 'Cho bac ky'? & Pre-75 Saigonese pronunciation.

 Hue Massacre , Tet 1968

What Ken Burns' Vietnam War docu-series on PBS gets wrong

Chó Bắc Kỳ = ''Northern [Vietnamese] dog''. Vietnamese south of the Ben Hai River 17th parallel use that term to pejoratively refer to North Vietnamese people (namely people in North and Bắc '75 (post-1975 North Vietnamese migrants to the South)). The term is usually used by Central, South Vietnamese when discussing:

*the Communist dictatorship (which is dominated and led by mostly Northern hardliners) and it's many injustices, crimes, amorality, incompetence, Chinese puppetry and other problems. 

More @ QUORA

*The article insinuates that it was only used after '75 which is incorrect as it was used at the time I first went to Vietnam in '67 though I only heard Bắc Kỳ which was derogatory none the less. I found this piece as I searched for  'Pre-75 Saigonese pronunciation' which I read in Saigoneeer and I assume that the Saigoneese accent has changed because of the influx of the northerners.......?