In a surprise shift in the social-media universe, Twitter co-founder Evan Williams stepped down as CEO on Monday and was replaced by COO Dick Costolo. In Williams' two years on top, Twitter has exploded from 3 million registered users to 160 million and from 20 employees to 300. He will stay with the company as "co-founder" (along with Biz Stone), but the question on every blogger's screen is: Why did Williams give up the reins? Here, four theories:

1. Twitter is ready for its "money printing" phase
Williams expanded Twitter's user base beyond tech geeks, attracting "normal people and celebrities and companies," says Dan Frommer at Business Insider, but the company needs to "pivot" from expanding its audience to translating its hit product into cash — and become a real business. Costolo is obviously the better man for that job. Look for "more advertisements [to] slowly creep into Twitter," says Ariel Schwartz at Fast Company. Costolo's challenge "will be to sneak in moneymaking schemes without turning off users."

2. Williams engineered the move so he could tinker
Williams, in a blog post at Twitter, says he asked Costolo to replace him so that he could be "completely focused on product strategy." Working on Twitter's new website, which was rolled out in September, helped him realize that "building things is my passion" and that he is "most satisfied while pushing product direction."

3. Twitter is preparing to go public
According to my sources, says Ryan Tate at ValleyWag, Twitter's board specifically sought out Costolo because Williams "was not generating revenue quickly enough." And Costolo "is said to be eager to take Twitter public." Clearly, Twitter is gearing up for an IPO, not a sale to a big buyer like Google (which bought both Williams' earlier startup, Blogger, and Costolo's, FeedBurner).

4. They are just formalizing their division of duties
"Though the titles are new," say Jeff Zeleny and Carl Hulse at The New York Times, Williams and Costolo had gradually "divided many of their responsibilities along those lines" long before Monday's shakeup. Once the dust has settled, Costolo will still focus on the business and revenue side, and Williams will still be working on new products, like the "new Twitter." Another sign the change was part of a larger plan: Costolo's first joking tweet as a Twitter employee a year ago: "First full day as Twitter COO tomorrow. Task #1: undermine CEO, consolidate power."