How would a manager use the progressive disciplinary action approach?

After reading chapter-11 and chapter-12 from the attached textbook answer the following questions. Answer should be in own words and strictly no plagiarism, no copy and paste from other resources. APA format must and answer in own words as thorough as possible.

Discuss three major factors that contribute to an employee’s decision to join a union. Discuss the five reasons that have contributed to the trend of decline in unionization.
How would a manager use the progressive disciplinary action approach? How would the use of this approach prevent employees from being surprised if they are terminated?