Update: Twitter has reacted to this raid by removing the term from its trending topics, but evidence can still be found on Twitter Search.
Twitter saw a huge influx of fake accounts on Sunday, pushing the NSFW trending topic, #gorillapenis, all the way to the top of the list. Popular image board 4chan /b/, along with other various sites, appears to be behind this latest raid on Twitter. 4chan, a hub of Internet culture and memes, has long been the spawning point of various raids on sites, most recently YouTube Porn Day. Before that, they were responsible for hacking Time Magazine's voting system for the World's Most Influential Person, making the site's creator, moot, the winner by a wide margin.
They are calling this latest effort "Operation Shitter." As you can see in the instruction set image, the blame is trying to be laid on Ebaum's World, which is, as far as we can tell, not involved in the raid. Blaming things on Ebaum's World is sort of a tradition on 4chan. A wiki (with information now pulled) was also formed to give instructions on how to systematically game Twitter's trending topics. The wiki suggests that raiders use a script, written by them, to spam the hashtag.
A similar NSFW trending topic on the subject of Mollie Sugden also sprung up recently after her death. While I'm not for censoring people by any means, when attacks like this pop up, Twitter has to have a better detection mechanism in place, ready to deal with it, especially as Twitter gets more and more mainstream attention.