You're looking at data collected by Google. You can probably guess that the lines on this chart represent how often the search terms are entered into Google. Note the gi-normous spike in late December 2010
January 2011 and the similar spikes in 2008 and 2009.
I'd say there's a pretty good chance that the spike was caused by ereaders given as gifts each Christmas. I'd also bet that most if not all got an ereader for the first time, otherwise they would have already searched for free ebooks.
Update: Guy just pointed out that I had the date wrong. The spikes aren't in January but are actually in late December. This actually makes the data fit better with what I've seen based on blog traffic.