Typically, it's classified as an unfair labor practice when an employer changes a collective bargaining agreement without the input of the union. However, recent guidance from the NLRB suggests that amidst the current economic climate, unilateral wage increases may be a justified reaction to labor shortages. At the start of the pandemic in 2020, the NLRB issued a memorandum that acknowledges an employer may be relieved of its duty to collectively bargain in the case of exigent circumstances. As such, employers whose businesses are threatened by an inability to hire may be able to use this to justify wage increases with the intent of attracting new employees.
See "Unilateral wage increases by employers may be justified by NLRB amidst severe labor shortages", Michael Pavlick, Bloomberg Law, October 24, 2022