Yahoo! Inc. CEO Carol Bartz sent a memo to employees Tuesday informing them that she had been fired by Chairman Roy Bostock over the phone — one of the more brutal corporate turnover tales in recent memory. "The company remains adrift" after Bartz's more than two years at the top, says The New York Times, despite Bartz-imposed "management shuffles, layoffs and the shedding of underperforming services." But what is the primary reason behind Bartz's sudden ouster? Here, four theories:

1. The company just wasn't performing
Bartz's termination should come as no surprise, says Ben Parr at Mashable. "Investors were unhappy with her performance." The company's stock has risen just $1.15 since Bartz took charge in early 2009, and the future didn't look any brighter. Yahoo simply "hasn't found a way to create new revenue streams, stabilize its finances, or retain its top engineering talent." With a track record like that, new leadership was inevitable.

2. She couldn't solve Yahoo's identity crisis
"More than anything, Yahoo couldn't decide what it wanted to be — a media company or a technology company," says Robert Hof at Forbes. "It's a problem that predated Bartz but one she of all people should have [resolved] once and for all." But she dithered on developing a clear message on what exactly makes Yahoo unique — while Facebook and Google galloped confidently ahead.

3. The Yahoo board is volatile
"Yahoo has done this rapid change of tune about its CEO before," says Shira Ovide in The Wall Street Journal. In late June, Bartz had the full support of the board; two months later, she's packing up her office. The same thing happened with former CEO Terry Semel in 2007. He was pushed out just days after getting a vote of confidence from then-Chairman Jerry Yang.

4. Key employees weren't happy with her
A number of top execs fled while Bartz was in charge, says Bloomberg Businessweek. And "one way of stopping the bleeding, the exodus of major people, is to find that she is the root of the problem," says Gartner's research analyst Allen Weiner.