The "one year certification rule" was originated in the early years of the National Labor Relations Board and has been consistently applied by it. Essentially it provides that after certification an employer is required to bargain with the certified union for a reasonable time, which is usually one year in the absence of "unusual circumstances." The certified union is conclusively presumed to represent a majority of employees in the unit for that period, the presumption afterward becoming rebuttable. This system of successive conclusive and rebuttable presumptions represents a compromise between the competing policies of giving a union time to establish a workable bargaining relation with the employer free from outside pressures and allowing a majority of employees to be represented by the bargaining agent of their own choice.
Eugene Alkema S.Ed.,
Labor Law - Certified Union's Loss of Majority Status During Certification Year and Without Fault of Employer as Justification for Refusal to Bargain,
Mich. L. Rev.
Available at: https://repository.law.umich.edu/mlr/vol54/iss1/5