Are Twitter users idiots who can't spell John Lennon's name right?
No doubt some of them are dumb enough to spell it "Lenon," which is currently listed as a trending topic.
So it appears that Twitter's trending topic algorithms ignore "Lennon" because the name of the greatest Beatle already shows up thousands of times every day of the year. In other words, the machine thinks Lennon isn't news. Clearly some tweaking needed.
This seems to parallel the situation with the nontrending Wikileaks.
Update: here's a good analysis of the Lenon/Lennon problem, concluding that "But it’s important to stop at this point and note that even though the reasons are understandable, they still make absolutely no sense."