Why is trade union membership a special category of data under GDPR?

I've been searching the internet for the answer to this question and come up blank, so thought I'd throw it out to you guys. I'm an American, and I can't imagine trade union membership being specifically called out in legislation as a sensitive category of personal information. I can postulate reasons why this might be different in Europe, but I was hoping someone could point to a specific circumstance or historical explanation for why trade union membership is considered a …

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