The spike happened one day after the pretty big earthquake (seismic intensity scale 4) on January 1, 2012, which caused the water level of the Skimmer Surge Tank of Reactor 4 to fluctuate unexpectedly.
Only the people on the net paid attention to the data and it was not reported by the MSM. The Ministry of Education and Science issued the result of the fallout measurement, without saying anything as to why these dates (12/27/2011 to date) are being singled out.
This person tweeted the pic with three overlapping charts comparing (from the top): (1) amount of snow on the ground and amount of rain, (2) amount of cesium fallout, and (3) wind speed (average speed in black, maximum speed in blue, and maximum instantaneous speed in red), from December 1, 2011 to January 6, 2012:
The huge spike looks a few sigmas away from the norm, even with the wind speed picking up on January 2, 2012.
On Twitter, people in Tokyo were reporting strange white powder floating in the air several days ago, and people in Ibaraki and Chiba Prefectures were reporting elevated air radiation levels.
No news from the media, and these reports are dismissed by some on Twitter as "hysteria".
Inage District of Chiba City in Chiba Prefecture also had cesium fallout sometime between 12/26/2011 and 1/4/2012, according to the analysis by the Japan Chemical Analysis Center: