Possible reasons sited are a "broken botnet", as well as the earthquakes in Asia, PCs being replaced with new ones during Christmas or simply being turned off and later breaking. They do however make note that many of these reasons would attribute for such a large dip in spam. They also note that the amount of spam is still alarmingly high, being in the high 80 percentile of all e-mail.
I guess we'll need to see what happens in the upcoming weeks and months to really determine how much was affected by what factors. But let's hope it drops even more!"