It's pretty shoddy journalism for the title of the article to assert this as fact even though it's just an allegation that hasn't even been investigated yet.
But they did investigate. They interviewed sources who confirmed that unionizing was planned before people were fired, which is the title. And they read the lawsuit and talked to the lawyers, who obviously said the same thing.
The title allows the reader to infer a causation that hasn't been proven or investigated. Given that the people making the allegation are bitter ex-employees who have a financial incentive for it to be true, it's an allegation that should be met with a healthy dose of skepticism.
You probably don't see the bias in the article because you support unionization. If the article was instead titled "Sexual harassment complaints double after NPM staff unionize," you'd probably be complaining about bias the same as I am.