The link:

Quality Coding Takes A Break For The Holidays. But Why?

The meat:

For the time period I looked at (the last 24 months), January through September is relatively flat and in line with the average flaw density. Then, there is a big bump in flaw density in October and November. Things begin to settle down once we go into December. The jump in application flaws is easy enough to spot. But what could cause this?

As a commenter points out, this probably has to do with deliveries schedule for Jan. 1. Seems like October and November are big push months with a lot of testing/debugging. December is polish and deployment.