http://www.tylervigen.com/spurious-correlations
In this case, the n-grams chart really exemplified this for me. There are so many influences on the frequency of a word, including lexical substitution of a word by its synonyms, changes in spelling, and increased or decreased interest in a topic regardless of whether that interest is positive or negative.
For example, check out the long-term decline in avarice in America! It's profound!
https://books.google.com/ngrams/graph?content=avarice&year_s...
Oh, wait, maybe we just stopped using the word "avarice" rather than the concept. :-)
Or, during this Second Gilded Age, our society actually started to become less atomized:
https://books.google.com/ngrams/graph?content=atomized&year_...
... or maybe we just moved away from calling the phenomenon that.