A lot of authors want to know what trends are affecting the public consciousness. They hope to tap in to a hot trend with a book that will create immediate interest in the buying public, whether that be a fiction or nonfiction book.
Popular "trendist" Faith Popcorn has created an entire career from pointing out trends.
An easy way to dip your toe in this murky water is to check out Google trends and/or create a search string of your own, plug it into your favorite search engine, and take note of the results.
By the time most people recognize a trend, it's ebbing. The trick is to be aware when it's building momentum.