“This was a personal decision of mine,” Day told analysts and the media during Monday’s conference call. “Plans have been laid for the next five years and a vision set for the next 10. Now is the right time to bring in a CEO who will drive the next phase of Lululemon’s development and growth.”

Day’s success as CEO is no secret. The company has a loyal following in North America and is now looking to expand globally, tasks that Day is suited for after serving as head of Starbucks’s Asia Pacific Group; she traveled 240 nights a year, running business operations in 10 countries in Asia. So why leave now? She claims it was her decision, and the much-discussed sheer yoga pants fiasco isn’t a convincing culprit.

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Perhaps her departure stems from a deeper dissatisfaction with Lululemon’s direction. “All of us in life want a sense of belonging and to know that we’re getting up and making a difference,” Day told CKNY AM 980 in October. “I believe in bringing all of yourself to work, and if there’s a difference between who you are and your values, and how a company operates, you’re always holding something back.”

While the board searches for a new CEO, it also has several senior positions to fill — including a new SVP of product operations and an SVP of logistics — and several new hires often lead to a new strategic direction.

In an email to Fortune, Day disputes that her departure is tied to the company’s five-year plan and 10-year vision. “There is no difference in strategic vision for the company, we were and are aligned.” Yet there’s certainly more to the story. Day continues, “My values include discretion. While I know everyone would like to know ‘the reason’ [I’m leaving] there are some things that should remain private because the truth is the good things outweighed the bad and by being respectful and grateful one can remember that.”

No matter “the reason” she’s leaving Lululemon, Day is no stranger to exiting a job on a high note. She left Starbucks (SBUX) in February 2007 at the top of her game — when she took over in 2004, the stock price of Starbucks Japan was in the low teens; when she left, it was in the low 50s — without another gig lined up. “I knew that I wanted to be a CEO. And I knew I couldn’t do it at Starbucks. Your first opportunity isn’t running a $10 billion company,” she told 
Fortune 
in 2008. Day promised her husband and three children she would take a year off — and though Lululemon founder Chip Wilson asked that she begin as CEO in October 2007, she honored her deal with her family (as did Wilson) and became EVP of retail operations in January 2008, and CEO that June. Day says she plans to stay at Lululemon for another four to six months and then plans to take six additional months off before “looking for my next CEO adventure.” Considering Day’s only 51 and has a relatively solid record with Lululemon, she could simply be hoping to find a new challenge.

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Day’s career decision may be right for her, but it has left the company’s board scrambling — and the company’s shares fell 17% on the Tuesday following Day’s announcement. She created a strong culture at Lululemon (last year, 70% of managers were internal hires), so it will be tricky to find a suitable replacement.

“I am very grateful for all who contributed to my success as a first time CEO,” Day says in her email to Fortune. “It was an incredible experience.” While she seems to be leaving the yoga-loving company in good spirits, one thing’s for sure: Lululemon’s board is probably feeling far from zen.