The employee was exposed to another employee who tested positive for covid-19. They asked him to stay home with pay for 14 days and he came back to the building to protest, putting other employees at risk.
From this distance, while we may all have our respective sympathies, both stories are plausible and we don't really know. It is abundantly obvious that companies generally find reasons (legitimate or otherwise) to fire those advocating for a union, but it isn't exactly unheard of for an employee knowing they are facing termination or disciplinary action for legitimate reasons to cover that over with some socially-acceptable reason like various claims of discrimination or starting a union like this, etc. It's not a secret hack nobody's ever heard of.
Edit: See boiled cabbage's comment: https://news.ycombinator.com/item?id=22739059 More data can bring more clarity.