What is forced Unionization?

You may be referring to "union shop" agreements. These are agreements between employers and employees that essentially require every employee to join and contribute a portion of his/her earnings to a labor union.

Under such agreements, new hires are informed that union membership is required as a condition of continued employment. New hires are typically offered a "probation" period, usually thirty days. If, after the probationary period, the new hire is acceptable to both employer and union, the new hire is immediately inducted into the union and his/her employment continues. If, at the end of the probationary period, the new hire is not acceptable to either employer or union, the new hire can be fired, that is terminated without recourse. At no time in this process does the employee involved have the opportunity to decide whether to join a union.

Union shops, prevalent in manufacturing plants during the 1950s and 1960s, have fallen into disfavor, and represent only a small part of the workforce in the US today.