Twitter has apparently “confidentially submitted” a filing with the U.S. Securities and Exchange Commission ahead of a planned initial public offering.

The company announced this news in a tweet, of course.

So why now? “They’ve gotten over about a billion dollars of investors’ money, and at some point, those investors want that money back,” says Marketplace’s Queena Kim. “And I think there’s a tipping point where if you raise enough money, you’re going to have to start making a lot of things public anyway…so you might as well go public and try to make more money.”

And Twitter’s revenue is already growing. “They’ve actually been doing all right with the advertising,” Kim says. “A lot of their money is coming from second screen advertising. So what this means is that as we’re watching the football game, people are talking about it on Twitter. Or if you’re watching the Academy Awards, people are talking about it on Twitter. That stuff is doing really well for Twitter.”

The word “confidentially” confused many when the announcement was made — not least becasue Twitter has more than 23 million followers, so the announcement was hardly made in secret. The Whiteboard’s Paddy Hirsch has a full explainer of what’s confidential about the IPO filing here.