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My first explanation would go something like this:
The change in mass caused by wax melting off the ends is what causes the candle to oscillate, and it is the rate at which the wax melts from either end which limits the size/speed of the oscillations.
The lower end will lose wax more rapidly than the higher end, as it is exposed to more flame, thus shifting the weight to the other side of the pivot point, i.e., the higher end.
(, Sat 12 Jan 2013, 12:16, Reply)