I have experienced that a couple of times, it is awful. I read through the posts here and a lot of it makes sense. I have always thought it was either related to the changing pressure on the descent or passing through the clouds - in the 3 or 4 times it happened to me, it seemed those factors were usually present - except for the time heading to JNB at 41,000 feet that I felt it a bit mid-flight. I think it also affects my sinus allergies so that when the allergies are at their worst, the pressure is worse.