Why do people from Barnsley hate people from the Nottingham area - calling them 'scabs'?

So as part of my university project, I went to watch Nottingham Forest FC vs Barnsley FC on New Years Day. There was about 2,000 Barnsley supporters in the away end and they kept chanting songs about 'scabs' and Margaret Thatcher but when I have seen Barnsley play numerous times at Derby County before (the team I support) there is never any chants like this. Why do they hate Nottingham and what the hell has Margaret Thatcher got to do with any of it ??