How to fix a self-turning handle on a Price-Pfister single handled faucet
Pfister is so aware of this problem they have a video on the problem http://www.pfisterfaucets.com/about/falling_handle.aspx

Turns out they made a faucets with heavy metal handles but cheap plastic internal parts... after a few years they fail.  I called up customer service, identified myself as an original owner, gave an approximate date of purchase, and was sent new parts.

They sent me a new metal nut, Pfister part 941-7110 . Then, they sent a new chromed plastic handle that has less mass.