ESPN first reported the news.
Butler himself has nothing to do with the complaint, per Breer. The NFLPA is allowed to move forward without his consent.
The NFLPA is filing the complaint on the grounds that club officials cannot tell a player that offseason activities are not voluntary, citing the current collective bargaining agreement. Butler was not allowed to participate because he was late to the first offseason training activity due to a cancelled flight, Breer reported earlier this month.
Breer notes that generally the union will ask for materials such as practice attendance and participation records, practice film, and discipline schedules in cases like this to see if there is a pattern and whether more than one player was disciplined.
When contacted by NFL Media on Thursday, the club declined to comment on the matter.