So say some Catholic scholars who have issued a document on the subject, according to CNS:

A group of Catholic scholars contends that management efforts to break labor unions are a grave breech of the church’s social doctrine and tantamount to committing mortal sin. A statement from Weymouth, Mass.-based Catholic Scholars for Worker Justice, released May 1, the feast of St. Joseph the Worker, offers a detailed argument that actions to thwart union organizing campaigns, stifle contract talks, unilaterally roll back wages and benefits, and break existing labor agreements are a “grave violation of Catholic social doctrine on labor unions. This violation of Catholic doctrine constitutes material grounds for mortal sin because it stands in grave violation of both the letter and spirit of Catholic social doctrine,” said the document, titled “Union Busting Is a Mortal Sin.” In laying out their argument, the scholars said efforts to deny workers the right to organize violate the First, Fifth and Seventh commandments regarding idolatry, scandal and theft, respectively. Joseph Fahey, professor of religious studies at Manhattan College in New York City and chairman of the scholars group, told Catholic News Service May 14 that the statement analyzes the criteria for mortal sin much like a priest would during the sacrament of reconciliation. “We said, ‘What commandments does (breaking a union) violate? What specific matters of Catholic teaching does it go against? Is it a grave matter? If it is, is there an objective case for mortal sin?” Fahey explained. 

UPDATE: A reader found the document on line.  You can read it for yourself right here.