This is new. Clicking on a Twitter trend, usually found in the right sidebar when logged into Twitter.com or on the homepage, you’ll notice above the search results a short explanation of exactly why that particular trend is relevant.
Up until now, there’s been one particular service called WhatTheTrend that’s sole purpose is to explain why a particular trend on Twitter was so popular. It’s still a pretty useful service, but not difficult to see how and why Twitter decided to incorporate the feature themselves into the service.
This seems to be part of a gradual redesign by the service that has been rolled out over the past few days. Subtle changes to artwork and a new default avatar were introduced earlier this week.
Thank you Simon!