Per the LA Times, which led me to read the Wikipedia article about the umpires union:
The MLUA, led by longtime negotiating attorney Richie Phillips, was decertified by its member umpires (by a vote of 57 to 35) in February 2000, after Phillips' strategy of mass resignations in September 1999 backfired, with MLB simply accepting many of the resignations and promoting new umpires from the minor leagues. The WUA was created immediately afterward. ...
The formation of the WUA allowed longtime umpires Derryl Cousins and John Shulock to finally join the umpires' union. They were denied membership by the MLUA due to crossing the picket line during the 1979 umpires' strike.