Some test data

This chart is for several recent year numbers. The area on the left is what
you'd expect. Growing mentions as the year approaches and continuing to
rise to a peak a couple of years later when (for example) a book about events
in 1993 actually gets written and published. It's the right-hand side that bothers
me. Notice the kinks at 2005 and 2007. After these points, it looks like
references to all 8 of the years started a simultaneous decline???
http://books.google.com/ngrams/graph?content=1990%2C1991%2C1992%2C1993%2C1994%2C1995%2C1996%2C1997&year_start=1990&year_end=2008&corpus=15&smoothing=0&share=