Education Week links layoffs to excessive employment of teachers over the last decade. From the article,

The NEA?s annual ?Rankings & Estimates? report shows that from 1999-2000 to 2007-08, student enrollment increased from 46.6 million to 48.9 million, an increase of about 5 percent. The number of K-12 classroom teachers during that time period rose from 2.89 million to 3.2 million, an increase of 11 percent. Figures over the same time period from the National Center for Education Statistics? Common Core of Data appear to confirm the pattern.

At this point, the link between layoffs and hiring is just a hypothesis, but it is one worth exploring.