This is something I heard from another good brewer, but I have never looked it up.
As the hops age, the Alpha acid goes down, I believe it transforms into Beta acid. Since hops are a preservative, they actually kill bacteria. Sour beer is made by adding bacteria. Aged hops have less of a preservative effect, thus the bacteria can sour up the beer.
MOD EDIT: Sig only takes up 6 lines now. Fermenting: Apple cider x 2, Doppelbock, Red, Oatmeal Stout, West Coast IPA x 2
Bottle conditioning: RIS
Keg conditioning: La Fin du Monde clone
On tap: Maudite clone, Double Munchen, Oktoberfeast, Oktoberfest Pilsner
Planning: Brewed in 2011: 22 (2010: 15, 2009: 29, 2008: 21, 2007: 1)
Wine made in 2010: 6 kits (2009: 19, 2008: 30)