Citing factors that included “an existential threat to a fledging union,” a federal judge Thursday ordered a private security firm to temporarily reinstate two guards who allege they were fired because of their organizing activities.In a written opinion, U.S. District Judge Thomas M. Durkin granted a petition by the National Labor Relations Board to require Universal Security Inc. to put Marcie Barnett and Sadaf Subijano back on the job at O’Hare International Airport.The preliminary injunction will …