Turbulence over Japan in Nov/Dec is a very common occurence (last year AA ICN-DFW and in 2008 one dead on UA NRT-HNL).
When you hear about this turbulence incidences, it usually happens at this time of the year somewhere around Japan.
Several jetstreams (coming off Siberia and Chinese tableau) seem to hit each other over Japan. Large temperature contrasts fuel the Jetstream.