My grandfather was born in Bedlington in 1913 to a single mother (of dubious morals IMO) and unknown father. He started his working life as a miner. In the 1939 census he was working in a hotel in central London. His wife (my grandmother), was working as a servant in a large house in the south. They left my father and his sister up here with relatives. One was left with Gr grandmother of dubious morals.
Grandmother came back to Bedlington. Grandfather was in WW2 and never returned, barely seeing his children again.
After the war he had a professional career, remarried, and although had no further children did very well.
I remember my great grandmother well and she had calmed down a bit in her later years, but often wonder if grandfather left Bedlington because his mother was a disgrace. That said, I hardly remember him. No birthday or Christmas cards. Nothing. So perhaps the apple didn’t fall far from the tree.
So although the mines were doing well, it may be personal circumstances that took your relative down south.