The entire scenario came up because of Walker's actions toward cutting some of the Public Employees unions' extravagant perks and getting his state back on budgetary track. In the size of his win -- mandate is not too strong a word -- Walker and Wisconsin voters told the public unions what they thought of them. And it wasn't only non-union citizens.
One of the laws that Walker had initiated which panicked Democrats so much was ending of the automatic collection of union dues by the government. Union members still had the option of voluntarily paying their dues. Well, money talks, and it also walks, and it walked away from the public sector unions. As Peggy Noonan wrote: "...the single most interesting number in the whole race was 28,785. That is how many dues-paying members of the American Federation of State, County and Municiple Employees were left in Wisconsin after Mr. Walker allowed them to choose whether union dues would be taken from their paychecks each week. Before that, Afscme had 62,218 dues-paying members in Wisconsin. There is a degree to which public union involvement is, simply, coerced."
When given a choice, even many union members reject unions.