Originally Posted by chong

Just probably because of a dip in temperatures. They may think that it's Fall again. If you over watered them, the leaves will have black spots (halves or whole leaves) in them, just like over fertilizing.

Which variety do you have? Chojuro is the hardiest and most productive, though not as tasty as the runner up. Followed closely by Nijissheki (Twentieth Century Pear).