The ancestors of the Sudeten German minority had settled there several centuries ago. So long as the Czech kingdom remained part of the Austrian Empire, the Sudetens, with their linguistic and cultural links with the ruling power, enjoyed a privileged position and high status, and tended to despise their Czech neighbours. Privileges included a near-monopoly of government jobs.
After 1919, in the new Czechoslovakia, the Sudetens felt sorely aggrieved - not because they were in any realistic way downtrodden, but because it is always very painful to lose ancient privileges. Hitler cleverly exploited these grievances to serve his expansionist aims. The mass expulsion of the Sudetens in 1945 was terribly unfair to the younger generation, who were not responsible for the deeds of their elders.
There may be a moral here for Unionist politicians who, against their better judgement, concede too much to extreme populist pressures.