If an employee is disruptive, hostile, negative and harmful to the environment for any other reason, of course they'd be fired, but if they're agitating for a union then it all becomes protected activity.
If an employee was working against the financial interests of the company in favor of the interests of a competitor, a supplier or a favored customer of course they'd be fired, but if it's a union then it becomes protected activity.
To be clear, I'm not making any judgement on unions or at-will employment or anything as being good or bad, or appropriate/inappropriate in any given situation, industry, etc. I just think it's super weird that from the perspective of the employer, you have an obviously cancerous group inside your organization which is actively working to harm you, but you're not allowed to defend against. It's weird.