I think it's pretty arbitrary, and no different than any other relatively close major cities. I'm in Calgary, and Calgarians hate Edmonton. New York rags on New Jersey ALL the time. EVERYONE hates Toronto. And so forth.
RTD is Welsh and simply ratcheted up the rivalry / playful antagonism between Wales and England like anyone would, albeit blunt to the point of confusion to us non-Brits. Doctor Who seems to have exorcised it with his departure, but since it's still shot there, traces of it will forever remain.