Control. Unions get a seat at the negotiating table. With a union things have to be done by mutual agreement, and the workers have recourse if things go wrong. Being a manager is much more powerful without a unionized workforce, finance notwithstanding.

They often personally take unionization as an insult or betrayal, so they try and get revenge using the one thing available to them at that stage. With overwhelming support, it’s only a matter of time before the vote passes and they are forced to recognize the union.

This is not uncommon. Managers want to rule over people without restraint even in nonprofits.