I would suspect that the right to do so was in the union bylaws.
Remember too that this was a small union; it was only in Wisconsin covering but one metropolitan area. Hence my point that a number of those voting to decertify might be seeing the need to join a larger union.
Never underestimate the potential for public school teachers to express leftist militancy.
This was point in my earlier post to you. Prior to Walker's reforms union members did NOT have the ability to force their leadership to re-certify. This was the 3rd largest union per the article and many others have done the same prior to this.
Always vigilant for sure but give credit where it's due.