[Brad Winder]

The short answer is that the paper inside turns acidic and that causes the cap to start allowing DC voltage through, basically like a resistor.

The long answer is that depending on what material the capacitor is built from, they all degraded at a different rate. Some manufacturers had issue within a year of manufacturing. Others had capacitors that lasted for many years with no issue. A good seal is crucial. But all paper capacitors degrade with time. The paper turns acidic, and the capacitor starts to leak DC voltages through. They become part capacitor, part resistor.