The secondary peaks 2-4 years out are interesting. Or perhaps it's better conceptualized as troughs the year after. Wonder what the reason is for that?
Thats clever! The longer a year has gone, the less likely you will say "last year", "one year ago", "two years ago" and rather go for the years "full name" like "2014". This could be a reason for the revivals.
http://chronicle.nytlabs.com/?keyword=2010.2000.1990.1980.19...
There is a little anticipation in the preceding years, then a big explosion in the year itself and then slow decline in the following years.