Shares of Baltimore-based Under Armour tumbled Thursday, as the market reacted to founder Kevin Plank’s plans to take over as CEO and replace Stephanie Linnartz after one year.

The athletic apparel maker said Wednesday after the stock market closed that Plank, who founded the brand nearly three decades ago, will return as president and CEO on April 1, more than four years after relinquishing the title. Plank’s return means the departure of Linnartz, who has led the company as both president and CEO for nearly 13 months.

Under Armour did not offer a reason for the management shake-up.

The surprise move sent shares tumbling as much as 13% Thursday. Under Armour’s stock closed at $7.23 each, down 10.7%.

Linnartz, former president of hotel chain Marriott International whose appointment last February was seen as a coup for the struggling brand, notified Under Armour employees of her upcoming departure in a note Wednesday. She did not offer a reason or mention future plans.

One analyst, Sharon Zackfia, a consumer analyst with William Blair, said in a report Wednesday evening that the abrupt departure of Linnartz after such a short tenure “likely points to some difference with Plank on the forward vision of the company.”

Plank is Under Armour’s controlling shareholder with a majority of the voting stock. “Culturally, Under Armour is a very difficult company to lead,” said Neil Saunders, managing director of GlobalData, in an email. “It still retains much of the DNA from its founding, and Kevin Plank has very distinct views about the brand and how the company should be run.”

Shortly after joining the company, Linnartz launched a three-year plan, Protect this House 3, to strengthen the brand in the United States, boost U.S. sales growth and elevate the design of footwear, women’s apparel and “sportstyle” products. The brand was viewed as behind rivals like Nike, Adidas and Lululemon in such areas.

The company’s stock has languished, mostly under $10 a share, since mid-2022.

A change in leadership so soon after Linnartz’s appointment shows the brand is unsure of its direction, Saunders said.

“Under Armour has already been through several rounds of change as it tries to address declining sales and issues with the brand, but, as the latest set of poor quarterly results show, it has not yet found a successful path to rebuilding the business,” Saunders said.

The key holiday selling season at the end of last year was mixed for Under Armour, which said in February that sales and profit fell in the third quarter, which includes the crucial holiday selling season. The company was forced to revise estimates for the full year, saying revenue is expected to be down 3% to 4% instead of the previously expected 2% to 4% decline.

Under Armour has underperformed for years and its stock price has plummeted 65% in the past three years.

The brand had been navigating a highly promotional retail environment as demand for its footwear in the U.S. has weakened. Meanwhile, it had begun working to revamp its leadership team, regain a reputation as a premium brand and make a splash in the sportstyle category.

Linnartz’s departure likely means additional shifts, analysts said, though Plank’s familiarity with the company should give him an advantage in trying to reverse the slides.

“All of the twists and turns have created a brand that has become increasingly confusing to consumers and to wholesale partners,” Saunders said. “This in turn, has made Under Armour easier to overlook. Remedying these problems are not simple, no matter who occupies the CEO seat.”

A filing with the Securities and Exchange Commission shows Linnartz, who will stay on as an advisor through April 30, reached a separation agreement with Under Armour on Sunday.

Linnartz stands to receive a $2.6 million cash payment, or twice her current annual base salary, a 2024 performance bonus and vesting of stock valued at around $7.3 million. The company will pay her lease on an apartment in Baltimore through mid-2024. The terms are based on her December 2022 offer letter and the company’s severance plan.

An analyst at Stifel credited Linnartz with bringing unique leadership skills to the job, putting in place an Under Armour rewards program and reshaping the senior executive team. Linnartz brought in a new chief commercial officer, chief communications officer, chief product officer, chief design officer, chief supply chain officer, chief marketing officer and other hires.

But she “had insufficient time to effect change evident in reported results, in our view,” said Jim Duffy, a managing director. “Under leadership of visionary founder Kevin Plank, we expect [Under Armour] continues to focus on high-leverage opportunities” such as brand visibility, product and sales in the U.S., Under Armour’s biggest and most challenging market.

Stifel maintained a “buy” rating on Under Armour’s stock, citing balance sheet strength and global growth potential.