From this: http://www.usatoday.com/news/health/2009-01-11-salmonella-peanut-butter_N.htm
Here’s the explanation from the outbreak linked to Peter Panpeanut butter a couple of years ago. Of course, that doesn’t mean this case is from a similar cause:
On April 5, 2007, ConAgra announced that its own investigation found that moisture might have triggered the growth of the Salmonella bacteria. Inadvertent moisture in the company’s Sylvester, Georgia, production facility could have allowed the growth of dormant Salmonella organisms that were likely present in raw peanuts or peanut dust, ConAgra stated in a press release. A ConAgra spokesperson stated that the moisture came from a roof that leaked during a rainstorm and from a faulty sprinkler system that went off twice during August 2006.