The NFL Players Association filed a non-injury grievance and a system arbitrator case on behalf of free-agent safety Eric Reid on Monday.
The union based its grievance on the premise that no NFL rule mandates players stand during the playing of the national anthem, that the league has indicated it respects "the rights of players to demonstrate," and the collective bargaining agreement states "league rules supersede club rules," according to a NFLPA press release.
The NFLPA asked representatives of free agent players "who had participated in peaceful on-field demonstrations to collect, memorialize and report any relevant information about potential violations of the [CBA] by teams," per the release.
Monday's move by the Players Association comes after Reid and his representatives last week filed a grievance under the NFL's collective bargaining agreement, alleging collusion that has denied him a job for the upcoming 2018 season.
"Colin [Kaepernick] and Eric have taken courageous action at the expense of their professional careers and personal lives," Reid's attorney, Mark Geragos said in a statement to NFL Network Insider Ian Rapoport. Geragos also represents Kaepernick in his grievance case.
"We stand shoulder to shoulder with the NFLPA in our fight for justice, equality and inalienable rights of all Americans."
The safety market has been remarkably slow in free agency this offseason. Veteran safeties Kenny Vaccaro and Tre Boston also remain unsigned. Reid has cited his social activism including kneeling during the national anthem as reason for him remaining unsigned. Reid has said he does not plan to kneel during the anthem.
In its Monday filing, the union alleges "a club appears to have based its decision not to sign a player based on the player's statement that he would challenge the implementation of a club's policy prohibiting demonstration, which is contrary to the League policy," and that "at least one club owner has asked preemployment interview questions about a player's intent to demonstrate."
The league did not respond to a request for comment.