Google started to show an OneBox at the bottom of the search results for the queries listed in Google Hot Trends. The OneBox mentions that a certain query is "#N of 100 most popular searches in the past hour", even though Google Hot Trends doesn't aggregate the most popular searches, which are pretty boring and don't change very often. As Google's blog explains, "Hot Trends lists the fastest rising searches on the web at any given hour".
Google uses the fact that a query is suddenly popular in various ways: for example, to include recent web pages in the list of top results or to show results from Google News and Google Blog Search. Unfortunately, Google Hot Trends doesn't do a good job at explaining why a certain query is popular, so Google should add more real-time news sources like Twitter, Flickr, YouTube.