Hardly a shocker. Post Act 10 a lot of public sector workers who were never crazy union membership took the opportunity to not have to pay dues. But it's also likely staunch union supporters have also decided it's not worth it anymore, given the limited bargaining power under Act 10.
Of course an argument would be made that now that dues aren't automatically deducted those who see the power of the union to restore what they had before Act 10 would see their dues as a good investment. Those who are still paying fall into that category. The number is telling.