In general, a move of two places in the rankings (especially when two schools were tied the previous year) can be attributed to statistical noise.
But in this case, there appears to be an explanation. I found this note in the ASEE database:
http://profiles.asee.org/profiles/7324/screen/19?school_name=Tufts+University
I borrowed a friend’s US News account and looked up the raw data. The field for “% in top 10% of class” contained “NA”. The previous year it was 90%.
Whatever value USNews choose to plug into their formula caused Tufts to be ranked as 57th for the “selectivity” portion of the ranking - 12 places behind Boston University. Without knowing the exact formula one cannot predict the impact on the overall rank, but I suspect that it would take off a few points, which would explain the drop.