HP has released an interesting report looking at where Trending Topics come from on Twitter. It claims that mainstream media are responsible for a lot of them. 

"You might expect the most prolific tweeters or those with most followers would be most responsible for creating such trends," says HP Senior Fellow and director of HP Labs’ Social Computing Research Group, Bernardo Huberman, but Huberman and three other researchers concluded that "user activity and number of followers do not contribute strongly to trend creation and its propagation."

"We found that mainstream media play a role in most trending topics and actually act as feeders of these trends," said Huberman. "Twitter users then seem to be acting more as filter and amplifier of traditional media in most cases."

Here’s the report:

The report finds tha 31% of tweets of trending topics are retweets. They collected data from Twitter’s API over a 40-day period, and from the resulting sample of 16.32 million tweets, they identified 22 users who were the source of the most retweets when a topic was trending." 

72% of those users were part of mainstream media outlets like CNN, the New York Times, El Pais and the BBC. 

Twitter itself recently explored the "science of the hashtag", illustrating how hashtag use can gain momentum.