Founder Kevin Plank is back at the helm at Under Armour (UA 0.61%) (UAA 0.75%), and investors aren't quite sure what to make of the development. Shares of the athletic apparel brand are down 11% as of 10:45 a.m. ET following news of the surprise CEO change.

A quick hook for the company's current CEO

Plank founded Under Armour in 1996 and led the company through most of its growth before stepping down four years ago. He will replace Stephanie Linnartz, who joined about a year ago after spending 30 years at Marriott International.

"As the company continues to navigate several post-pandemic consumer, industry, and brand-specific factors, we are working hard to reconstitute our strengths and make thoughtful, balanced business decisions to drive enduring success for athletes, customers, and shareholders," Plank said in a statement. "I am energized about the team we have put into place and look forward to seizing the opportunities ahead."

The timing of the announcement appears to have caught investors off guard. Linnartz was in the early stages of a three-year turnaround for the brand, taking steps including launching a loyalty program and growing Under Armour's women's apparel offerings. She will remain an advisor to the company through the end of April.

As part of the transition, Mohamed A. El-Erian has been named non-executive board chairman. Plank, who had been executive chair of the board since stepping down as CEO, will remain a director.

Is Under Armour stock a buy?

Wall Street tends not to like surprises, and investors are understandably on edge following this move. Linnartz was generally well regarded, but the move would appear to imply that her strategy was not gaining traction as investors had hoped.

Plank's first time in charge was marked by rapid North American growth, a strategy that will not be easy to replicate in 2024. At best the move would imply that the hoped for turnaround for a stock off 85% from its all-time highs will take longer than expected. At worst, there is more bad news on the horizon.

Investors are understandably taking a wait-and-see approach from the sidelines.