Annalee Newitz from the science fiction blog io9 says,
We did a study (with related analytical chart!) of how many zombie movies got released every year since 1910, created a line graph of them, and correlated spikes in zombie movie production with social unrest. It was actually quite eerie, because wars and social upheavals always seem to be followed by a giant leap in numbers of zombie movies in theaters. Coincidence? You be the judge!