Founder Kevin Plank is back at the helm at Under Armour (UA -11.38%) (UAA -12.98%), 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 CEOPlank 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 ...