Example: there currently exists a bishop who will be the next pope. His web profile today is substantially smaller than BCC's, and is likely dominated by 200 word wiki articles, a page at his diocese, and perhaps some Time Magaxine profile from when there was speculation as to who would follow JP2.
His Holiness will eventually pass away, which is going to ignite an immediate Worldwide News Event that will cause query volumes for the top ten candidates to skyrocket. Google wants to return fresh results for those (e.g. not the pages currently ranking). Using Twitter to trigger QDF gets that hours before the link graph will reflect the new speculation articles' popularity, without requiring manual intervention.
This is, of douse, just one signal. Google News feeds, the query spikes, and sudden radical increased appearance of branded terms on trusted pages all point to the conclusion.