Looks like something to do with taxes, but I can't figure out the phrase that matches his trends. "Taxes", "income tax" and "tax forms" all produce trends that have similar peaks as his, but nothing has that distinctive rise and dip at the end of the year.

Neat idea. This seems like the sort of thing that could be done with genetic algorithms - the problem has a natural objective criteria and evolving a search string is relatively straightforward.