This was some time ago, right? She's not in her late 30's or anything, is she?
(might be a clue)
People do it all the time though. Committment to different people means different things and is important only in certain situations.
Maybe her experience wasn't as positive as yours and something happened to push her over the edge after she had initiated?
My mother turned in her pin after her big and the older girls graduated. She had nothing in common with her pledge class b/c she had spent the last 3 years hitchiking and studying in Europe while those girls had just graduated HS. Sounds perfectly reasonable to me.
The bottom line was that she probably wasn't enjoying it and felt that her time and emotional energy were better spent elsewhere. Greek life ain't for everyone.