According to the article your statement is not entirely correct:
"The Wisconsin Budget Repair Bill required public unions to hold an annual recertification vote."
Curious - when was the last time the unions held a re-certification vote?
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.