"Article 3.
Recognition, Union Shop and Checkoff
...
Section 7. Supervisors Working
(a) The Employer agrees that the function of supervisors is the supervision of Employees and not the performance of the work of the employees they supervise. Accordingly, the Employer agrees that supervisors or other employees of the Employer who are not members of the bargaining unit shall not perform any bargaining unit work, except to train employees or demonstrate safety, or as otherwise provided in the applicable Supplement, Rider or Addendum.
However, in the case of Acts of God, supervisors shall comply with the procedures in subsections (b) and (c) and may only perform bargaining unit work until bargaining unit employees are available. The Employer shall make every reasonable effort to maintain a sufficient workforce to staff its operations with bargaining unit employees. The Employer also agrees that supervisors or other employees of the employer who are not members of the bargaining unit shall not perform bargaining unit work in preparing the work areas before the start of the Employer’s hub, preload or reload operation, nor shall the Employer send any bargaining unit employee home and then have such employee’s work performed by a supervisor or other employees of the Employer who is not a member of the bargaining unit."
Grieve it. You'll win. And provide your supes with a salutory lesson as well.
As to what they could legitimately have done... What was your start time and what breaks had you been given? What State are you in? In some situations they could have put you, while waiting, on break or unpaid meal break... but what they actually did was a no-no.