http://economics.csusb.edu/faculty/nilsson/personal/Professional/Nonnested.pdf
Here is an academic paper (economics) discussing union decertification and providing data. A graph in this paper shows the striking rise in union decertifications since about 1970. According to this paper, less than four union certifications occur for every decertification, over the past few years. I found whole how-to books on decertifying unions. Apparently it is a growth industry.