15K Feet altitude Drop

A while back I read about a commercial jet liner cruising along at around 35K feet and all of a sudden lost altitude of around 15K feet but recovered and landed safely. Does anyone know why it lost that much altitude quickly? There has to be some logical reason as dropping that much altitude in such as short time don't feel good with the passengers. Funny also was that I have not heard any of the passengers interviewed about their experiences.