More companies are keeping track of their former employees through things like alumni networks. As companies have trouble finding workers with the right skill sets, rehiring a former employee can be an attractive option. According to the Bureau of Labor Statistics, professional and business services hired the most workers in August, but also had the highest number of job openings at the end of the month. Studies have also found that rehired former employees have higher retention rates.
See "Boomerang Employees", Melissa Korn, The Wall Street Journal, October 24, 2011